details for motorways and trunk roads. Examples are shown in Figures 23.13 to 23.16. A Concrete Society working party reported in 1974136 on a wide range of edge details for all types of roads. The report covers function, design considerations, design practice, construction, maintenance and relative costs.

Column foundations. The column foundations, whether planted or bolted-base, must always include duct routes for the incoming and outgoing cabling which should take account of the allowable bending radii of the cables. The column foundations should also be designed to allow the column door to face away from oncoming traffic whenever practical.

23.10 Lighting, signing, communications and safety

Feeder pillars. Feeder pillars should always be located so as to allow safe access for maintenance. The feeder pillar foundations should also include ducts for incoming and outgoing cables, as described for columns, above. Spare ducts for future cables should be included wherever practical.

23.10.1 Lighting Lighting can be expected to produce significant savings as a result of a reduction in night-time accidents, and upgrading old installations can also be expected to reduce accidents. The provision of street lighting in urban areas can deter criminals and there may also be an amenity value. There may be some environmental intrusion in rural areas, depending on the location. The need for lighting on trunk roads and motorways should be appraised in accordance with TA 49/86.l37 A cost-benefit calculation and assessments of environmental and road-safety factors will generally be required and a framework approach can be adopted. Street lighting may be required for safety reasons alone at such locations as follows. (1) (2) (3) (4) (5) (6) (7)

In urban areas subject to a speed restriction of 30 mph. In semi-urban areas subject to a speed restriction of 40 mph. Where the road layout is substandard. Where junctions occur at frequent intervals. At roundabouts. At grade-separated junctions. On fog-prone sections of road.

23.10.1.1 Design principles The design of all aspects of lighting is covered in BS 5489.l38 The aim of any road-lighting system is to illuminate all areas and aspects of the road and traffic which are of importance to all users, including pedestrians, in a manner which is aesthetically pleasing, especially in areas adjacent to areas of historical or visual interest. The main steps of the design process may be summarized as follows. (1) The road category and preferred lighting arrangement (i.e. one side, staggered, etc.) should be decided. (2) The data required for the calculation using BS 5489 and manufacturers' data should be compiled. (3) The design spacing should be calculated. (4) The lantern positions on the road plan with respect to fixed features such as junctions, pedestrian crossings, etc. should be plotted. (5) The remaining lantern positions on uninterrupted stretches of road based on the design spacing but 'grading' into the fixed-feature locations should be plotted. (6) The overall layout should be checked for misleading nighttime arrays, general daylight appearance and 'line-of-sight' obstructions. (7) The column locations should be checked on site for obstructions and visual acceptability. On the basis of the site survey, minor adjustments should be made as necessary. 23.10.1.2 Practical considerations In addition to the design procedures described above, the following practical requirements relating to the installation of the lighting columns and associated equipment should be borne in mind.

Cable routes. Cables should be buried 600 mm below ground level and should be laid on 75 mm of sieved sand and covered with a further layer of sieved sand to a depth of 75 mm. Cables should be run in soft ground wherever possible and the route of each cable must be marked with purpose-made cable markers indicating the cable voltage, depth of burial, joint positions and changes of direction. Ducts. Where it is not possible to provide cable routes in soft ground, ducts should be provided, of minimum size 75mm diameter. Typical locations where ducts are required are road crossings, cable routes within concrete structures, cable routes across vehicular accesses, etc. Duct routes should take account of the permissible bending radii of cables and should include cable drawpits at all changes of direction and at intervals of 35 to 50 m, depending on the size of the cables involved. Cast-in conduits. It is necessary in some instances (e.g. where lighting fittings are fixed on the underside of overbridges) to cast-in conduit routes from a location adjacent to the underground cabling to the light fitting(s) via a suitable chamber for location of the fused cutout unit normally located in the column base. 23.10.1.3 Maintenance and operation The maintenance and operation of lighting systems is discussed in the British Standards.138 Developmental Standard TD 23/86139 deals with these aspects on trunk roads. 23.10.2 Traffic signs and road markings A congress held in Vienna on 8 November 1968 resulted in the Convention on Road Signs and Signals. This included recommendations for signs and signals and set out standards for them. Further meetings were held at Geneva on 1 May 1971 and on 1 March 1973. These resulted in the European Agreement supplementing the Convention on Road Signs and Signals and the Protocol on Road Markings respectively. These provisions have been accepted and agreed by the European Conference of Ministers of Transport and the European rules concerning road traffic, signs and signals140 co-ordinates the provisions of the above. The UK provisions are laid down in Traffic signs regulations and general directions 1981^ and subsequent amendments and guidance on use, size, siting and illumination of signs and details of road markings are given in the Traffic signs manual40 and Circular roads No. 7/75.141 Further information is contained in Department of Transport circulars, standards, advice notes and a comprehensive bibliography,142 which includes publications relating to working drawings for traffic sign design and post sizes, is produced also by the Department. Permanent traffic signs, excluding signals, can be divided into:

(1) Warning signs, to advise of hazards (2) Regulatory signs, to advise drivers of legal restrictions. (3) Informatory signs giving information about directions, routes, places and facilities. Road markings give information to help the drivers to select lanes and not as a guide base day and night. They may give a warning, a requirement, or a notice of restriction of prohibition. For night-time visibility, signs may be illuminated externally or may contain reflective material, although in urban areas the use of one or other requires special consideration. The use of reflective material in road markings is a valuable aid to visibility as is the use of reflective road studs. Variable message signs are also increasingly used as part of traffic control at locations where a message is not required to be displayed permanently or where alternative messages are required at different times. Examples of their use are as part of lane control, speed control, detection and warning to overheight vehicles and warning of adverse weather conditions. They can either be of the mechanical type (e.g. roller blind, prism or flap) or wholly electric where either an internal light source reveals the message or of the matrix type where an array of light sources create a range of messages, through the use of fibre optics. Details of road signs and markings and a comparison of their use in many countries of the world were contained in the report of the Permanent International Association of Road Congresses143 to the Geneva congress held in 1973. 23.10.3 Communications 23. JO.3.1 Motorway standard equipment All motorways in the UK are equipped with a standard Department of Transport specified communications system which includes electric cables, roadside equipment cabinets, signals, mains power supplies and electronic equipment. Practice and presentation are consistent throughout the country. The system provides emergency telephones from the motorist's viewpoint, giving direct connection to the police in the local motorway control room, together with signals which indicate either a general need for caution or for specific actions, such as speed restrictions or changes of lane. All motorways are each equipped with a multipair communications cable along one side only, taking a route as far from the carriageway as possible and which forms the longitudinal cable, which when connected to others at motorway-to-motorway junctions, provides a national cable network used by the motorway computer centres and control offices to communicate with each other and with the equipment to be controlled, such as signals, emergency telephones and certain lighting systems. These cables, in their various sizes, cater for all normal requirements in a motorway communications system.

which has various reflective labels both on the outside facing the oncoming traffic and on the inside of the housing door. These labels show the reference number and letter of the telephone, which the motorist must give to the local motorway control office when reporting his situation. Various standard logos are also part of the telephone housing labels. Telephone systems are normally, initially, a stand-alone system installed ready for motorway opening and comprising separate groups each of up to sixteen telephones and controlled by a telephone bridging unit near the roadside which sends the group's calls over a rented line to the motorway control office. The phase II systems which follow later operate the telephone under central computer control, switching them locally on to loaded circuits in the longitudinal communications cable and hence to the motorway control office. 23.10.3.3 Motorway signals The most basic signal system, called Motorwarn, is that installed temporarily to cover from opening until the permanent signals are operational. Each signal is separate, consisting of a post set in concrete carrying a pair of amber lanterns, a radio receiver and a flasher mechanism which causes the lanterns to light alternately with a period of about 1 s. Power is supplied from a car battery on the ground below and the signal is switched on or off by radio transmission from an approaching police patrol vehicle. Permanent motorway signals take two forms, depending on whether it is necessary to apply speed restrictions equally to all lanes of a two-lane or three-lane carriageway, or to apply different restrictions to each of any number of lanes. The former system is known as 'carriageway signalling' and employs postmounted matrix-type indicators sited in the central reservation, while the latter is known as 'lane signalling' and mounts similar signal indicators on a gantry, with one over each lane. Access to a motorway at a junction with an all-purpose road, may be controlled by post-mounted matrix signals having additional red flashing lanterns. All permanent signals discussed above have locally mounted units, either within the mounting post or on the gantry, known as 'distributors' and 'controllers' which are integral parts of signals. Mains supply is from Electricity Board interface cabinets usually set in the boundary fence and feeding signal installations over standard pattern mains-supply cable. Control of signals and transmission over the motorway longitudinal cable network until the relevant responder is activated is from motorway control offices via the central computer. The responder is mounted in a cabinet near the roadside, local to the signals under its control, and acts on instructions bearing its electronic address. It also signals to the computer when any of its emergency telephones are off-hook and switches them to the motorway control office when instructed.

23.10.3.2 Motorway emergency telephones

23.10.3.4 Current developments

These are provided at approximately 1.5km intervals along continuous motorway, normally in pairs, with one for each carriageway. This ensures that a motorist can reach a telephone by walking along the carriageway without attempting to cross it. Extra pairs of telephones at motorway junctions are sited on the route but within the junction to accommodate motorists isolated by the sliproads. In the case of motorway-to-motorway interchanges, the link roads and main routes are provided with as many telephones - but not necessarily in pairs - as are required to meet the criteria stated above, which preclude the crossing of a carriageway or a spacing greater than approximately 1.5km. The telephone instrument is mounted in a standard housing,

All the above information refers to the National Motorway Communication System 1 (NMCS 1), but a new system, NMCS 2, is being brought into service using distributed computers associated with motorway control offices. The cabling to signals and other controlled devices is slightly different but uses similar cable with more cores in the longitudinal cable. Mono-mode fibre-optic cable is coming into service as a standard cable for closed circuit television, but could easily be used to carry long-distance communications in the future. New services such as the automatic traffic surveillance system, fog detectors and ice detectors are coming into service with NMCS 2 and can be easily connected into the existing longitudinal cable at the nearest cabinet.

23.103.5 Traffic signals The use of traffic signals is dealt with under sections 23.6 and 23.12.

the limiting consideration is that the noise passing through the barrier should be less than that diffracted over or around it. The minimum mass required is given by the expression: M= 3 x antilog ^~^°^ kg/m2

23.10.4 Fencing 23.10.4.1 Boundary fencing Motorway boundary fences are owned and maintained by the Department of Transport. It is normal, to simplify supply and maintenance, to use one type of wooden fence or one type of strained wire fence with droppers (Figures 23.35 and 23.36). The latter is less obtrusive than the former and would be used, for example, at the top of cutting slopes to reduce the effect on the skyline. Other permanent fences generally become the responsibility of, and are chosen by, the landowner after erection by the Department of Transport. They are selected, if possible, from the accommodation fences included in the Highway construction details*2 or BS 1722.144 Where appropriate, deer fencing is provided as shown in Figure 23.37. Timber for use in the works and preservative treatment for timber fences should comply with BS 5589145 subject to amendments detailed in the Specification for highway works.12

where A (taken as positive) is the potential attenuation in dB(A) calculated by the path difference; and M is the mass in kg/m2. 23.10.4.3 Safety fences and barriers

Noise barriers can be made of glass reinforced polyester, glassfibre cement, plastic-coated steel, aluminium, etc. as well as of conventional materials such as timber, brick and concrete. The material should be chosen to suit the surroundings. Grassed or planted earth mounds are aesthetically the most acceptable, particularly in rural areas. Criteria for design should be based on technical memorandum H 14/76 and Amendment No. I. 146 The main principles to be borne in mind are:

A safety fence absorbs some of the energy caused by a vehicle striking it and redirects the vehicle. A safety barrier provides containment and vehicle redirection without itself being deflected or deformed. The main types of safety fences and barriers used in the UK are: (1) tensioned beam safety fence (corrugated or rectangular hollow section);42 (2) untensioned beam safety fence (corrugated or open box);42 l47 and (3) British concrete barrier (concrete profile barrier).147 Use of untensioned corrugated beam fences and concrete barriers is restricted to low speed roads (85 km/h or less) and in the case of British concrete barriers is limited to urban areas. Safety fences are provided on the central reservations of all new motorways. They may also be used on dual-carriageway trunk roads and other all-purpose roads where there are high traffic flows or obstructions such as bridge piers and sign gantries. They are also used at the back of verges on embankments 6m or more high, and on other embankments where there is a road, railway, water or other hazard below, the outsides of curves of less than 850 m radius on embankments over 3 m high, and at obstructions such as bridges, large signs and retaining walls. The requirements for safety fences on new and existing trunk roads are detailed in TD 19/85.148

(1) The barrier should be sited close to either the noise source or the position to be protected for optimum effect. It should be long enough to obscure completely the noise source from view at the observation point. (2) Normal range of barrier height is between 1 and 3 m. Less than 1 m is ineffective and more than 3 m usually unacceptably intrusive. Greater heights may be achieved by erecting a barrier on top of an earth mound. (3) Comparatively light material is usually sufficient because

23.10.5.1 Motorway service areas Department of Transport policy was to provide service areas at intervals of about 40km, with potential infill sites midway between. The land, connections with the motorway, access roads, landscaping, parking areas, lighting and basic services are provided by the Department. The operator, who leases the site from the Department, pays for the buildings and equipment

23.10.4.2 Noise barriers

23.10.5 User facilities

38 x 87 rails nailed to each post with 2 no. 4 x 100 nails

Backfill square 300 (minimum)

Main post 75 x 150 x 2100 long

Concrete grade C 1OP to this level unless otherwise directed

All dimensions in millimetres Figure 23.35 Wooden-post and five-rail fence. (After Department of Transport (1987) Highway construction details. HMSO, London)

765 long dropper 650 long dropper

minimum

Holes 300 diameter augered or minimum 200 square Intermediate post 1975 long 40 x 40 x 3 RHS

Strut 1975 long 40 x 40 x 3 RHS 450 square or 450 diameter augered hole Training post 2275 long 60 x 60 x 4 RHS AM dimensions in millimetres Figure 23.36 High-tensile strained-wire fence with droppers. (After Department of Transport (1987) Highway construction details. HMSO, London)

765 long dropper

925 long droppers

minimum

925 long droppers

Minimum 450 square or 450 diameter augered hole concrete C 1OP

400 wide x 450 long concreted OP .Double strut 3025 long 50 x 50 x 4 steel RHS

Straining post 3025 long 100 x 50 x 4 steel RHS maximum spacing 300 m All dimensions in millimetres Figure 23.37 High-tensile strained-wire deer fence. (After Department of Transport (1987) Highway construction details, HMSO, London)

Intermediate post 2875 long 50 x 50 x 3 steel RHS Minimum 200 square or 300 diameter augered hole concrete C 1OP

and is required to maintain the site and buildings in good repair, including the facilities provided initially by the Department, and to provide services for every day of the year, including toilets, food and drink retail outlets, and fuel and repair facilities, although in some cases this latter requirement has been relaxed. A committee of inquiry into motorway service areas produced a report in 1978149 which reviewed existing facilities and their use and made recommendations for the future. 23.10.5.2 Laybys On rural all-purpose roads the provision of laybys is recommended at the rate of two per 1.6 km, although it is acknowledged that the spacing will be dependent on topographical features and the horizontal and vertical layout of the road. Details of their layout and advice on siting are contained in Layout of roads in rural areas]5Q and the metric supplement.151

23.11 Specifications and materials testing 23.11.1 Introduction The Department of Transport Specification for highway works12 covers all aspects of works on highways (except signals), bridges and other associated structures. There are seven parts, and the first six are divided into twenty-six series, each covering a separate aspect of the work and an introduction. The final part covers accepted quality assurance schemes, certification-marked quality assurance schemes, approved lists of proprietary systems and materials, publications referred to in the Specification72 and the manner in which variations to the specification can be made by adding, deleting or substituting clauses. Specific criteria may be inserted in appendices to each section. Clauses may include the terms 'or as otherwise described in Appendix *****' or 'as described in Appendix *****'. Other terminology used is 'as described in the Contract' or 'as shown on the drawings'. The additional information must be added for such clauses to become effective. Where clauses include the terms 'unless otherwise agreed by the Engineer' or 'as approved by the Engineer' the engineer is given freedom of action to suit particular circumstances. The complementary document, Notes for guidance on the specification for highway work73 is in six parts which relate to the first six parts of the Specification72 Each part is divided in a similar manner to the Specification.72 The Specification is mandatory for use on Department of Transport schemes and is used generally for other major roadworks in the UK; it usually is incorporated by reference in road and bridgework contracts. A specification may be 'end-result' in which only the finished work is tested or may be by 'method specification' in which the method of working is defined. Some blend of the two types of specification may be adopted. 23.11.2 Control testing Control testing may be defined as testing with the aim of monitoring compliance with the specification requirements for the material in question. It is principally carried out by a contractor or supplier and may be predictive in nature. The test may be the specified test itself or some other test which correlates sufficiently well with the specified test. 23.11.3 Acceptance testing Acceptance testing is the testing carried out to ascertain whether

compliance has been achieved within the specified test limits. This may involve tests on the components of material prior to placement, or tests on the material after placement checking both the material content and workmanship standard. 23.11.4 Statistical acceptability In circumstances in which materials are produced as a continuous or semi-continuous process, specifications may require a statistical method of testing aimed at ensuring that early corrective measures are taken to prevent the required material parameter falling outside the specified range. Examples of the application of the specification are as follows. 23.11.4.1 Earthworks Cuttings Tests are taken primarily to determine whether the excavated materials are suitable for forming embankments. Acceptable limits for some of the tests are laid down in the specification but in others the designer is required to decide the limits. For cohesive materials, tests that may be required are grading, plastic limit, undrained shear strength of remoulded material and either moisture content or moisture condition value. A normal permitted moisture content for acceptability is not less than optimum moisture content (Test 12 of BS 1377)74 and not greater than 1.2 times the plastic limit. The test for granular materials may be grading, uniformity coefficient, moisture content or moisture condition value, and for chalk, the saturation moisture content. A normal permitted moisture content for granular materials for acceptability is 1% from optimum to 2% below the optimum as determined by the laboratory compaction test (Test 12 of BS 1377).74 Embankments. Compaction is normally controlled by a method specification, although the engineer is permitted to carry-out field dry-density tests for comparison with similar tests on approved work in similar materials to confirm the compaction applied. 23.11.4.2 Concrete and cement-bound materials Concrete is tested in its hardened state by cube crushing.152 Nondestructive testing is used mainly for comparative purposes to identify defective areas of completed work.152 Aggregates and cements are tested separately.153"155 23.11.4.3 Bituminous materials Bituminous materials are tested for their composition156 and compaction from cores by calculating the percentage refusal density as the percentage ratio of the bulk density of the sample to the final density after compaction to refusal. 23.11.5 Method of measurement A method of measurement157 is used on Department of Transport, and other, major roadworks contracts based on the Institution of Civil Engineers Conditions of contract.*™ It is also based on the Specification for highway works12 and the Highway construction details.*2 The method allows bills of quantities to be prepared in a uniform manner for the benefit of engineers and others closely associated with highway works. The Library of standard item descriptions^ is also provided. Variations in the conditions of contract, the specification or highway construction details will require amendments to the method of measurement to suit the variation. Details of the amendments so made are required to be stated specifically.

23.12 Roads and traffic in urban areas 23.12.1 Introduction Roads and traffic in urban areas are parts of the complex arrangement and use of the urban fabric. The growth of traffic has caused congestion, particularly in the peak hours and has had an impact on the environment and the people who live and work in urban areas with the noise and air pollution associated with traffic and increased danger. Much has been done to combat these problems. Urban motorways and other major roads and improvements have been provided, but the general tendency is now to make better use of the existing fabric by providing traffic-management measures by a variety of means to reduce congestion and delays: (1) trafficcontrol measures have been introduced; (2) priority has been provided to public transport; (3) through traffic has been removed from residential areas and shopping centres; (4) pedestrianized areas and special facilities have been provided for cyclists and pedestrians as well as the disabled; and (5) safety has been improved. New roads can still contribute to the general good of the area and enable these improvements to take place more easily and, whilst new roads can have local adverse effects, they can still provide substantial overall benefits if properly sited and designed. Roads and traffic in urban areas*2 provides a comprehensive review, and is intended as a guide to good practice in dealing with urban area problems. 23.12.2 Hierarchy of roads It is most useful and desirable to establish a hierarchy of roads within an urban area, whereby a policy can be developed for the use of each road. There is an interaction between the highway network and land use, and the latter can be controlled in relation to its position in the road network. Major through routes, for example, are primarily for traffic use and the number of accesses to it should be strictly controlled. Residential roads, at the other end of the spectrum, should have all through traffic excluded. The various levels of distributor roads can be decided and it may be possible to make the physical characteristics of the road more appropriate to its place in the hierarchy. It may be possible to channel more traffic on to main traffic routes thus allowing heavily used shopping areas to be pedestrianized. 23.12.3 Traffic management Traffic management is aimed at improving an existing road network to meet set objectives, without having to resort to substantial new construction. Such objectives may include some, or all, of the following: (1) a reduction in road accidents; (2) environmental improvement; (3) improved access for people and goods; and (4) improved traffic flows on major routes. These aims can be achieved by employing various traffic management measures involving combinations of: (1) Improved road capacity. (2) Giving priority to certain classes of vehicles, e.g. buses, emergency vehicles and cyclists. (3) Restraining the demand for road space by introducing some form of restrictive measure. (4) Improved facilities for pedestrians, cyclists and the disabled. Traffic management measures may be limited to a small, localized area or may encompass an area-wide network. In the latter case, it should be recognized that measures regarded as provid-

ing solutions for one area can create problems elsewhere. It is therefore vital that all proposals be properly evaluated before implementation, and that adequate data collection and forecasting techniques be employed to examine effects over an appropriately wide area. The Association of London Borough Engineers in association with the Department of Transport and the Metropolitan Police have produced a code of practice for traffic management in London.160 The guidelines and advice contained within the code may have useful applications elsewhere. 23.12.4 Methods of assessment The potential traffic throughput and effectiveness of alternative traffic management measures can be assessed by traffic simulation modelling techniques, such as CONTRAM,18 TRAFFICQ,19 SATURN,17 and TRANSYT58 referred to earlier in section 23.4. Such models typically require input data which includes link lengths and widths, junction type, traffic flows and estimates of the origins and destinations of the traffic pattern surrounding the study area. Other computer models such as ARCADY,49 PICADY,49 MIDAS50 and OSCADY54 can be employed for the detailed assessment of individual junction designs within a scheme as outlined in section 23.6. New roads may be proposed or new roads in conjunction with traffic management measures might be suggested as a package, and these proposals can be compared with other possible trafficmanagement measures. The assessments should be made with the aid of a framework described earlier in this chapter so that all aspects may be considered. As with inter-urban roads, public consultation and participation is becoming more widespread and the results can be included; the impact on the environment and people is a major issue and both benefits and disbenefits will almost invariably arise in an urban area. All other appropriate factors should also be included. Any specific improvements in traffic and financial benefits in cost-benefit terms arising therefrom are, as in inter-urban roads, only two aspects of the comprehensive summary which must be produced to allow decisionmaking. The SACTRA report9 and the government response10 has been mentioned earlier. 23.12.5 Improvements in road capacity The urban road network consists of a complex interaction of road links and the junctions which join them. Measures aimed at improving road capacity can therefore be assessed in terms of link or junction improvements, either separately or in tandem. 23.12.5.1 Road links The treatment appropriate to road links between junctions should reflect the extent to which they serve the functions of through movement, local distribution and access to frontage premises. Link improvements often may be achieved by straightforward measures such as road widening, or by the imposition of waiting and loading restraints161 or turning restrictions. Other forms of link improvement commonly adopted include tidal flow arrangements and the creation of one-way streets. 23.12.5.2 Junctions The traffic capacity of a road system is commonly constrained by its junctions. An important consideration when junction improvements are being investigated may be whether the existing junction type should be maintained, perhaps with modifications, or whether a different type of junction control would be

more appropriate. Factors influencing the decision will include: (1) the existing and predicted traffic flow and composition; (2) the classification and function of the roads forming the junction; (3) available highway space; (4) proximity of adjacent junctions; (5) road safety record; and (6) other needs, such as public transport, pedestrians and cyclists. The four main types of junction in common use are: (1) (2) (3) (4)

Major/minor. Signalled. Roundabouts and gyratory systems. Grade separation.

Major/minor junctions. Major/minor junctions are the most common type of junction in urban areas, their great advantage being that major road traffic is generally not delayed (except traffic waiting to turn right into the minor road). Improvements to heavily trafficked junctions may include the channelization of traffic to separate and clarify conflicting movements. This may be achieved by improved road markings and signing or by physical means, such as traffic islands. Departmental Advice Notes TA 23/8142 and TA 20/8448 give the main principles for layout and recommended design standards as stated previously. Major/minor junctions tend to become unsuitable when minor road traffic cannot find suitable gaps in the main road traffic during times of peak traffic flow. These conditions can result in excessive queuing and delays in the minor road, often leading to accidents or diversionary routing. In these cases, an alternative form of junction control will be necessary and traffic signals often prove beneficial. Signalled junctions. Traffic signal control is an important feature of junction control in urban areas, providing relatively efficient control within the confines of limited road space. Traffic signals initially were provided to reduce the police manpower required to control traffic. The use of traffic signals to control traffic movement can now bring about major reductions in congestion, improve road safety and enable specific strategies to be introduced which regulate the use of the road network. Such strategies might be: (1) to reinforce a designated route hierarchy; (2) to give priority to public service vehicles; (3) to provide crossing facilities for pedestrians and cyclists; and (4) to maximize traffic flow. Section 23.6 includes details of computer programs used in connection with traffic signals and Department of Transport advice notes on this subject. New signal controllers using microprocessors are now available for use at isolated junctions. Information on the traffic approaching from each leg of the junction can be processed at the signal controller and the signal timings adjusted to provide a more efficient use of the junction. The numbers of traffic signal systems in urban areas may be extremely high, leading to adverse interaction between neighbouring installations. Recent advances in computer technology have made it possible to co-ordinate the operation of adjacent traffic signal sites by the use of area-wide traffic control.12 This in turn has enabled vehicle movements to be controlled over a section of road network, which may result in a reduction in vehicle journey times, the number of stops, fuel consumption and environmental pollution. This is the basis of the majority of present-day urban traffic control schemes. Many different programs are in use: fixed-time programs such as TRANSYT16 58 adjust signal timings based on historical data, but SCOOT162 both receives data on traffic and adjusts signal timings on real time. Other potential benefits which an urban traffic-control scheme may provide include: (1) the implementation of diversion schemes and variable message systems; (2) creating priorities for buses and bus routes; (3) priority for emergency vehicles

responding to incidents; and (4) provision of special signal timing plans to favour key routes from fire or ambulance stations. Roundabouts and gyratory systems. Roundabouts provide a useful form of junction control which is generally conducive to 'free-flow' conditions when they are operating within their capacity limitations.49 The different types of roundabouts, their capacities and designs have been discussed earlier in this chapter. They do not require control equipment and are therefore not prone to equipment failure. When approach speeds are very high or some of the approach links are dual carriageways, roundabouts are often the safest way of regulating traffic. In addition, they provide good opportunities for vehicles to turn right and allow U-turn manoeuvres, which can be very beneficial if restrictions exist elsewhere. However, roundabout solutions are unsuitable where linking of traffic flows between adjacent junctions is advantageous or where it is beneficial to change traffic priorities at different times of the day. The provision of pedestrian and cyclist facilities may also prove difficult. In some circumstances (e.g. such as a measure to reduce accidents or during peak periods) there may be advantages in signalling one or more approach paths to a roundabout. Gyratory systems - whereby a series of one-way streets are linked to form a circulatory system - can overcome the limitations of several small junctions. However, they can create access problems to properties within the central island. Grade separation. As grade-separated junction facilities are quite expensive and can be visually intrusive they will generally only be used at junctions of major importance, where land is available and there is little or no adverse environmental impact. 23.12.5.3 Priority management measures Traffic management measures may be undertaken to provide priority for particular classes of traffic, e.g. public transport vehicles, emergency vehicles, cyclists and pedestrians. Priority measures are normally achieved by allocating special facilities such as pedestrian62 and cycle crossings, bus/emergency vehicle actuated signals163 and bus and cycle lanes. Alternatively, certain classes of vehicles may be exempted from general traffic restrictions, e.g. exemption from turning bans or contraflow operations. Priority measures are often applied where there is a general deficiency in the road network in terms of demand related to capacity. Any such measures should seek to ensure that the overall effects provide a net gain to the community as a whole and, where possible, should attempt to minimize the adverse effects on nonpriority road users. 23.12.6 The provision of information The efficiency of traffic management schemes can be enhanced greatly by the careful provision of traffic signing and roadmarking systems. Well-designed, clear and comprehensive signing should provide drivers with information on route choice well in advance of the approach to any junction and also warn of any turning prohibitions. Such systems are likely to be enhanced greatly by the emergence, in the foreseeable future, of electronic route guidance systems163 such as AUTOGUIDE, a trial of which has been proposed in London, and ALI-SCOUT which has been installed in an area of West Berlin on a trial basis. A trial on route guidance, the CAC system,143 was undertaken in Tokyo some years ago. In addition, the use of signing can provide information on items such as car-parking availability, diversion

routes, guidance to tourist attractions, and direction signing to cyclists and pedestrians. The use of signs and markings has been dealt with earlier in this chapter. 23.12.7 Restrictive measures Traffic restraint (or demand management) measures may be employed to control the level of traffic in an area or on a particular route. For example, in one area restraint may be aimed at through-traffic and in another area at commuter movements. Restraint measures are designed to encourage those making trips to respond to imposed conditions in specific ways. Various responses may include: (1) a change in the mode of travel; (2) a change in the time of travel; (3) using a different route; (4) travelling to an alternative destination; or (5) not making the trip at all. Very few restraint schemes have been implemented in the UK other than those involving some form of parking control164 mainly effected through on-street waiting restrictions165 or lorry bans.166 However, more radical measures have been attempted in other countries such as Singapore, where there are restrictions on entering the central business district in the morning peak hours, and in Hong Kong,163 where a trial on the feasibility of road pricing has been undertaken. 23.12.7.1 Facilities for pedestrians and cyclists Pedestrians and cyclists can be very vulnerable to severe or fatal injuries on the roads and it is essential that traffic management measures take their safety into account. Typical facilities may include: (1) separate provision of footways or cycle routes; (2) guardrails; (3) pedestrian crossings; (4) subways or footbridges; and (5) pedestrianization schemes. In addition, consideration needs to be given to the handicapped.167 Where space is limited, facilities for pedestrians can sometimes be shared with other users. 23.12.7.2 Residential areas Previous reference has been made to residential areas. The residential precincts in The Netherlands (Woonerf) have been the subject of much interest.163 The demonstration projects in The Netherlands163 adopted three strategies for residential areas: (1) the exclusion of through-traffic by simple means; (2) the exclusion of through-traffic by simple means compiled with small-scale measures for reducing speed; and (3) the conversion to a residential precinct. It was felt that, for most areas, preference should be given to a redesign to the second strategy which gives relatively the best results for the costs involved. 23.12.7.3 Parking Parking should be considered within the overall policy relating to the infrastructure and use of the urban area. The demand for parking, particularly in central areas, often exceeds the available space. The generous provision of parking space is likely to encourage the use of the private car and the limitation on the amount of parking to be provided may act as some deterrent to users of private vehicles. Parking may be provided on-street, or off-street in groundlevel or multistorey car parks. On-street parking may be controlled by indicated time limits free of charge or some payment may be required such as at parking meters. Parking charges, both on- and off-street, usually vary and depend on the location in relation to the central area. Residents' parking schemes are often provided where parking spaces in an area would be otherwise occupied by the cars of

visitors or commuters to the area. Charges may be levied or permits to park may be provided without charge. Enforcement is a major part of parking policy. Fines are usually levied when the permitted parking time is exceeded. Vehicles may be removed in certain circumstances, and wheel clamps are now being introduced in certain areas which, in addition to incurring financial penalties, cause drivers delay and inconvenience before the cars are freed and can be driven away.

23.13 Highway maintenance 23.13.1 Introduction As outlined in section 23.2, local highway authorities normally act as agents for the secretaries of state in maintaining trunk roads and motorways, in addition to maintaining their own highways. The object of maintaining a highway is to preserve the fabric in such a condition that it provides safe passage for all traffic throughout its life. The Report of the committee on highway maintenance*6* proposed that maintenance operations be divided into: (1) structure; (2) aids to movement and safety; and (3) amenity. It is not easy to ascribe priorities within those groups without assessing the type of traffic, its contribution to the community and the advantages to be gained by incurring the expenditure, or even the disadvantages, of not doing so. The report168 proposed national maintenance standards for the UK; good maintenance extends the life of a road pavement and adds to the convenience and safety of the public using the road. Good maintenance means: (1) Day-to-day maintenance to maintain a road in proper condition for the traffic using it. This includes patching, surface dressing, gully emptying, repairs to drainage, kerb and footway maintenance, maintenance of bridges and other structures, embankments and verges, repair and maintenance of traffic lights, carriageway markings, street lighting and street furniture, snow and ice clearance. (2) Structural work required to extend the life of the road or to enable a road to carry an increased volume or weight of traffic. Tables 23.17 to 23.19 set out the needs in relation to structural works, aids to movement and amenity. Regular inspections and a system to determine priorities for short- and long-term attention (i.e. a maintenance rating system) will be required. Most highway authorities carry out regular detailed inspections of highways at specified frequencies and have recording systems for defects which require maintenance action within a specified period of time. The Department of Transport has instituted a code of practice169 setting maintenance standards with a routine maintenance management system applying to motorways and trunk roads which sets out requirements for a computerized inspection system which is linked to an inventory of the highways and produces routine maintenance work schedules. The Department also specifies the statement of service170 and detailed standards to apply to the winter maintenance activities for motorways and trunk roads. The local authorities associations in the UK have produced a code of good practice for highway maintenance171 with model specifications of maintenance activities as a guide to the preparation of highway maintenance policies and standards. Each highway authority produces particular maintenance policies to apply within the area, including those policies and standards relating to winter maintenance.

Table 23.17 Structural elements of highway maintenance Element ( ) Item

Correction

Remark

Carriageway (I)

Condition Shape Irregularity

Reconstruction, resurfacing surface dressing

Depending on degree of fault

(2)

Strength

Reconstruction, resurfacing

Depending on degree of fault

Surface dressing

This is corrected within other corrections operations as above

Surface dressing

To maintain a watertight surface

Surface (I)

Lack of skid resistance only

(2) Patching (3)

Defect through utility services

These should be masked during surface operations (Road markings are entered in Table 23.17)i

Drainage (I)

Gully emptying

(2)

General performance

Width of running water in channels

Standards relate to the flow in channels reducing the effective width of carriageways

(3)

Concentrated flow across carriageway

Install extra channels or gulleys

Not to be confused with normal flow across cambered roads

Dependent on local conditions - referred to also under sweeping and cleaning

(See also 'Structures' below) Footways (Urban) Irregularities related to safe use

Relay paving slabs

(1)

Suggestions for height of projections are given and also rates of inspection in different areas

Resurface flexible material

(2)

Legal implications must be recognized

Footways (Rural) Irregularities and surface water related to Minimum maintenance in rural areas safe use

Rate of inspection given

Kerbing (1)

As drainage feature

Install to delineate drainage channel and support edge of carriageway

(2)

Edge of footway

Defective kerbing

A normal height of kerb in urban and rural conditions is given as safety feature for pedestrians

Kerbs sunk to carriageway level or lower

Serves to support edge of carriageway

Necessity for programming of painting of steelwork

Rate of inspection to be assessed on local conditions

Structures (1)

Bridges, culverts, walls

Table 23.17 - continued Element ( ) Item Safety of road user is paramount

Correction

Remark

Deterioration of concrete or other fabric Underwater inspection where necessary

(2)

Embankments and cuttings; to include ditches where appropriate

Regular inspection for incipient slips or failures Advice to adjoining landowners about structural or drainage defects

Legal implications are important

Correction

Remark

Road Markings (I) Advisory markings

Remake

General recognition should be possible

(2)

Mandatory and statutory markings

Remake

These markings must lie within legal limits

(3)

Reflecting studs

An inspection before onset of winter Loose or ineffective studs should be conditions is advisable and equally an replaced. A wholesale change might be inspection should be made after winter made at the end of effective life maintenance operations are complete

Table 23.18 Aids to movement and safety Element ( ) Item

Traffic signs and bollards (I) Illuminated signs and bollards

Regular inspection for (i) light failure (ii) drainage (iii) cleaning (iv) supports and frames

Particular attention to be paid to mandatory signs, e.g. 'Stop' signs, etc.

(2)

Nonilluminated signs

Regular inspection for (i) drainage (ii) cleaning (iii) supports

See above

(3)

Traffic signals

Regular inspection for (i) light failure (ii) general maintenance and cleaning (iii) phasing (iv) alignment (v) mechanism (vi) painting

(1) All highway personnel should report faults wherever discovered (2) Contract maintenance and guarantees to be operated. On-call arrangements to be made

Pedestrian Crossings (I) Beacons (2) Road markings Road Lighting (I) Lanterns

As for traffic lights (i) Slippery surfaces should be corrected Legal implications are important (ii) Obscure markings should be made good Regular inspection for (i) illumination (ii) cleaning Conditions to be reported during inspection

The safety of the road user - driver and pedestrian - is paramount in this connection

Table 23.18 - continued Element ( ) Item

Correction

(2)

Conditions to be reported during inspection

Columns

Guard Rails and safety fences Pedestrians and vehicular

Winter maintenance (1) Precautionary salting

To be included in regular inspections. Where risk to public is involved speedy action is necessary. Inspection to cover: (i) condition (ii) painting (iii) cleaning

Remark

Note legal implications

On receipt of frost warning roads should Treatment should be applied within a limited be treated period after a warning: The rate of salt-spread should be 14g/m 2 (a) Rural main roads and motorways - 2 h Salt in accordance with BS 3247: 1970, (b) Other important roads and accesses to Part 1 emergency services - 2 h (c) Urban main roads - 1 h Crews of salting equipment should be on stand-by duty. Neat salt should be used. This should be mixed with grit in special circumstances only

(2)

Snow and ice clearance

(3)

Snow fencing

Local knowledge is necessary to establish the siting and timing of snow-fence erection. Care must be taken to clear this with landowners

(4)

Salt storage

Care must be taken to site salt heaps to avoid damage to local crops, watercourses

Use of specialist equipment

Major routes as in (a) above should never become impassable to traffic. This is related to traffic flows which have the effect along with salt of keeping snow from accumulating. Snow ploughs with blades effectively remove slush from main routes; crews should be on standby duty. Roads as in (c) above should not be impassable for longer than 1 h. Public transport is a major factor in the clearance of snow. Roads in other priorities should generally be cleared in 4 to 6 h unless conditions are exceptional. Pedestrian ways may require special treatment in town centres. Footpath clearance should be confined to busy areas, steep hills, etc.

Table 23.19 Amenity items Element ( ) Item

Correction

Remark

Grass cutting

Standards vary for urban or rural situations

Grass cutting (I)

Prevention of obstruction of sight lines

(2)

Maintenance on certain roads of reasonable pedestrian access

(3)

Control of noxious weeds

Rural (a) Major roads - 1 .8 m of the verge should be kept below 15 cm. Elsewhere one or two cuts should be employed to keep grass to 30 cm long (b)

Minor roads - one cut per year will normally suffice

(c)

Spraying of grassed areas can be used to control noxious weeds. Consideration must be given to width of road, nature of traffic and not least, the culture of the grass growth

Urban (a)

Major roads - grass should be kept down to 8 cm

(b)

Minor roads - minimum maintenance consistent with the environment.

Use of chemical sprays

These should be used with caution where access for cutting is not available and for growth control generally. Examples are: around sign supports; central reserves with safety barriers; urban walkways

Tractor-mounted equipment might be employed

This is not normally a function of the highway authority. They have power to require land owners to reduce hedge heights and to control trees. Legal implications should be noted carefully

Hedge Trimming Prevention of obstruction of visibility at bends and at traffic signs

Trees This repeats very much the information under hedge trimming. Trees may call for specialist advice which is often available in the Parks Department of a Local Authority

Legal implications and ownership are important factors

Sweeping and Cleaning (1)

Objects and material shed by vehicles

Heavy and dangerous items must be removed by hand This is material which can cause broken windscreens and mechanical damage

The rates of inspection and activity on this account vary with the weight of traffic, between daily inspection on motorways and weekly visits on less busy roads. All highway staff should be aware of the necessity to remove dangerous items whenever they see them

Table 23.19 - continued Element ( ) Item

Correction

Remark

(2) Vegetation and detritus

This is material which can block drainage Legal implications are important systems which can obscure road markings and cause dirty windscreens Again rates of inspection and activity are dictated by traffic but where traffic is heaviest, cleaning and scavenging is difficult. In rural areas carriageway sweeping is not necessary more often than at 2-monthly intervals. In town centres daily attendance is required, reducing to weekly attention in residential areas. This should include footpaths. The rate of gully emptying is dependent on the build-up of such material. In dry weather it may be necessary to top up gulleys with water to permit drainage traps to operate. This can fit in with road-washing operations

The structural needs of the road including resistance to skidding, surface irregularity, changes in traffic and usage of the road and a 'maintenance' rating168 for, say, each 500 m length of road, requires to be derived. Computer programs have been developed to handle maintenance ratings for an extensive road network. 23.13.2 Assessment of structural needs Two methods of assessing structural needs are the CHART 172173 and MARCH174 systems. Maintenance assessment surveys are required by the Department of Transport in the determination of structural maintenance needs of motorways and trunk roads. The CHART and deflection surveys assist in the identification of priorities for structural carriageway maintenance. The CHART is a visual and computerized inspection method of recording specific defects which, with the aid of a computer model, evaluates the condition of the road and recommends types of treatment with priorities. Deflection measurements of flexible pavements are primarily taken by deflectograph, which is now used extensively on motorway and trunk road surveys. It is supplemented as necessary by deflection beam measurements.175177 The deflection beam is usually used on short lengths of road of 1 km or less. The deflectograph provides a more rapid method of measurement than the beam and is better-suited for routine surveys of long lengths of road. Skid resistance is checked in the UK using the sideways force coefficient routine investigation machine (SCRIM) developed by the TRRL. This provides a printout of sideways force coefficient (SFC) at 10m intervals along a road and can test about 1500km per year.178 I8° 23.13.3 Aids to movement and safety Items falling under this heading are the routine requirements for signing, lighting and similar items and the winter maintenance requirements which have to be met as they arise.

23.13.3.1 Amenity items Items included under the general heading of the amenity functions of a road also have a safety element in that grass-cutting, tree-lopping, etc. ensure that sight-lines at bends and the visibility of traffic signs are maintained. 23.13.3.2 Maintenance organization The organization on the ground ranges from the outdated individual or lengthman method to the more cost-effective mobile gang/team. The latter method utilizes mechanical equipment with specialist teams for surface dressing, patching of carriageways, road markings, lighting, signing, bridge maintenance, motorway maintenance, etc. 23.13.3.3 Winter maintenance Setting-up a winter maintenance system involves determining the standards to be adopted and then examining the meteorological data for the district so as to establish the period during which the winter maintenance organization will be expected to function. The organization with its resources and plant will undertake de-icing, gritting, snow clearance and similar work. 23.13.3.4 Signing for highway maintenance Signing for maintenance work on UK highways is covered in the Traffic signs manual.™ Information in the Traffic signs manual for major roads is amended and supplemented by Departmental Standard TD 14/83.m This standard includes the TRW series of drawings which detail roadwork-signing requirements on motorways and dual carriageways including contraflow traffic control. For minor roads (unclassified roads which form about half of the total road mileage in the UK), Department of Transport Advice Note TA 47/85l82 deals with the control of traffic at roadworks on single-carriageway roads and is complemented by

Department of Transport Standard TD 21/85183 concerning portable traffic signals at roadworks. All signs must comply with the Traffic signs regulations and general directions.53 Staffordshire County Council, with the assistance of eleven other county councils, have produced a useful 54-page booklet entitled Safety at roadworks.***

out by the foremen or inspectors and sometimes by members of their teams. The 'general services' available for maintenance work could include, for example: (1) a signs store, traffic signals, etc.; (2) a plant depot; (3) a winter maintenance depot and salt stores; and (4) workshops. An 'emergency' team will comprise a pool of skilled labour which can be used for special operations.

23.13.3.5 Staff for maintenance

23.13.3.6 Alternative staffing

The type of staff required for maintenance includes those: (1) for inspection, estimating and programming work; (2) for superintending day-to-day operations; and (3) for carrying out the physical work, either by direct labour or contract. The officer controlling highways in a local authority will encompass the whole range of operations: (1) provision of new highways and bridges and their physical effects on the country through drainage arrangements, earthworks, etc.; (2) the maintenance of the roads, and its management in terms of control of traffic; and (3) surveillance of operations of public utilities and of security generally. He will probably require a second-incommand over the whole range of interests if the total length of highways is in excess of about 3000 km. Figure 23.38 illustrates a typical organization. The Report of the Committee on Highway Maintenance168 indicated that about 800 km of road would form a useful area unit; this could be increased significantly if comprehensive support services were readily available. The area surveyor will be responsible for all maintenance in the area and will have to deal with some of the design problems met from time to time. It will be necessary to interpret survey data provided as a basis for work programming and to deal with the financial implications. The area superintendent will need to organize labour and undertake ground surveys for work-programming purposes. The number of foremen must depend on the nature of the area and of the men under their control, e.g. rural conditions vary from urban conditions. Many of the inspections will be carried

The Local Government Planning and Land Act 1980 requires local authority direct labour organizations to compete with contractors in the private sector in tendering for highway works contracts of maintenance and construction. Some local authorities have in consequence set up separate contracting organizations similar to the public works contractor within the local authority. The structure of such a highways department is divided into client and contracting organizations. On the client side, area surveyors/engineers with technical and administrative support staff work 'on the ground' and are responsible to the maintenance officer at headquarters. Contract works managers with direct-labour resources and staff, responsible to the works officer at headquarters, operate in the field to the client's requirements. The contractors in these cases also provide the highways emergency services, including winter maintenance for the authority. The Department of Transport provides facilities for their agents.

23.14 Low-cost roads in developing countries 23.14.1 Introduction This section comprises only an outline guide to the basic principles of road design and construction in developing countries. The nature of roads in these countries varies widely

Chief Officer Deputy (when required) Assistant New Highways Not developed in this section

Assistant Highway Maintenance Area Area Area Area General Surveyor Surveyor Surveyor Surveyor Services Assistant Surveyor Area Superintendent

Area Foreman

Area Foreman

Area Surveyor

Figure 23.38 Typical operational structure for highway maintenance staff

structure

Emergency Team

depending upon the particular geographic, economic and political situation of the country concerned. Some countries follow British design and construction practices, some follow practices in the US and others those relating to yet other countries. Most countries will have developed road standards of their own, often based on a blend of one or more of the above but taking into account local conditions and requirements, availability of local materials and other local aspects, and it is essential that the designer should refer to the particular standards relating to the country in which he is operating. A publication produced for UNESCO185 covers all aspects of design construction and maintenance of these roads. Roads in developing countries have a number of common characteristics which influence the approach to design and generally require a philosophy different from that in the developed world. These characteristics include: (1) (2) (3) (4)

Long lengths of road under consideration at one time. Low traffic volumes. Limited finance. Traffic composition (ox carts, bicycles, overloaded lorries, etc.). (5) Tropical climate and geology (intense seasonal rainfall, lateritic soils, etc.). (6) Poorly trained workforce and limited availability of construction materials.

The design and method of construction of a new road will vary significantly depending on the above characteristics and also on considerations of cost-benefit analysis, funding, and strategic and political factors. A solution should be selected (whatever the circumstances of a particular scheme) which is appropriate to the type and volume of traffic, geology, climate and availability of labour and machinery. The construction of new roads in a developing country is a vital element in opening up the countryside and boosting industrial and agricultural development. Finance frequently is provided, at least in part, by an external funding agency. The economic return initially may be small in relation to the large capital expenditure and therefore it is generally of prime importance to keep the construction cost to the minimum. This can be achieved in a number of ways: (1) Phased development: it may be appropriate to construct a gravel road in the first instance with the aim of adding a surfacing as traffic volumes and finance become available. (2) Appropriate construction techniques: this may involve labour-intensive techniques in countries with a large labour force, but mechanization can often be a more appropriate solution. (3) Appropriate design standards: these in respect of highway geometry, drainage and construction. 23.14.2 Route development Routes for roads will often follow existing tracks but where a new alignment is to be provided the following principles will generally apply: ( I ) A route should be chosen to avoid structural and drainage problems. (2) A route should be chosen to avoid excessive earthworks. (3) The alignment should be kept simple, as simple curves and straights are easier to set out in the field. Land boundaries and property acquisition will not generally be a limiting factor in developing countries. (4) Where possible the vertical alignment should be slightly above ground level to aid drainage. Where this is not

possible drainage considerations are of paramount importance. (5) The route should, where possible, be aligned near sources of locally available construction materials. Suitable mapping often does not exist or is unavailable and it may be necessary to rely on aerial photography and walking the route to fix a line. The road geometry will vary depending on the volume and type of traffic and the terrain. The cross-section, in general, should incorporate a verge at each side. This enables vehicles to pull off the road and also protects the edge of the pavement construction from erosion and damage. Figure 23.39 shows typical cross section standards adopted in tropical countries and also typical standards for low-volume roads. 23.14.3 Drainage Water is the main enemy of road construction particularly in tropical countries where rainfall is frequently very intense. As already stated, drainage of the road formation is of paramount importance and is best achieved by setting the road slightly higher than the surrounding land and providing adequate sidedrains as shown in Figure 23.40. Only in urban areas is positive drainage appropriate. Drainage should be simple. Side-drains will generally discharge directly into streams or on to surrounding fields. Sidedrains of appropriate cross-section are easily maintained by the use of a grader. Only on steep gradients need the ditch be lined with stone pitching to prevent erosion. Cross-drainage should be kept to a minimum but where required is best achieved by the use of simple concrete pipes or corrugated steel structures. Complex reinforced concrete structures are not appropriate in developing countries except for long-span bridges. A work by Watkins and Fiddes186 deals with rainfall runoff and highway drainage. 23.14.4 Pavement construction Many different types of pavement construction are used throughout the world and the type chosen will depend on the forecast volumes of traffic, the finance available and other factors specific to the country in question. The pavement in its simplest form will consist of an earth track, the existing soil being graded to a smooth running surface. Many tropical soils will perform well provided they are well shaped and adequate drainage is provided. A modified surface can be used if local soils are unsuitable or if traffic volumes demand a stronger road. This can be achieved either by stabilizing the existing soil in situ to improve its properties or importation of another suitable soil from nearby. Imported soils of suitable characteristics for road surfacing are typically natural gravels or crushed stones with a significant clay content. Existing soil can be stabilized by many techniques. For example, cement may be added in small quantities (4 to 5%) to strengthen the soil, or lime can be added, which has the effect of reducing the plasticity index. The quantities to be used must be determined by trials. Single-size and wind-blown soils are notoriously difficult to deal with but have been successfully stabilized using bitumen; this is expensive and other organic products such molasses may be appropriate in certain countries where such agricultural byproducts are plentiful. Gravel roads under heavy traffic flow require sealing to provide improved riding quality and protect the surface from the ingress of rainwater and axle load damage. The simplest

Side drain Shoulder Flat terrain

Carriageway

Shoulder

Side drain Mountainous terrain

Existing ground level 0.6 m minimum

Surface seal

Sub-base Base

0,6 m minimum

Stone-pitched side drain

Formation

F I at terra in Rolling or hilly terrain Mountainous terrain

Average daily traffic >1500 7.3 Paved width ^L1 (m) Shoulder width L2 (m) 3 Design speed (km/h) 100 4 Maximum grade % Shoulder width L2 (m) 3 Design speed (km/h) 100 Maximum grade % 6 1 Shoulder width L2 (m) Design speed (km/h) 80 7 Maximum grade %

Paved class 500-1500 6.7 2-3 100 5 2 80-100 6 1 60-80 8

50-500 6.1 2 100 6 2 80 7 1 60 8

Grave/ class 50-500 < 50 6.1 5.5 2 1 80-100 60-80 6 8 1 1 60-80 50-60 8 10 50-60 10

Figure 23.39 Typical cross-section of a low-cost road

Bad practice

Earth from roadway and ditch

Water from ditch and surrounding country draining on to road. Road deteriorates rapidly

Good practice

Figure 23.40 The importance of good drainage, (a) Good practice; (b) bad practice. (After Watkins and Fiddes (1984) Highway and urban hydrology in the tropics. Pentech Press, London).

Earth bladed from side drains formation kept drained. Road life prolonged

30-50 12

form is a bituminous seal.188 This can be upgraded to a double surface seal or a thin asphaltic concrete surfacing if the economic benefit can be shown. In some parts of the world, other materials such as waterbound macadams and penetration macadams, are employed. These can be appropriate if labourintensive techniques are to be used. Pavement design is undertaken by an empirical method as with other roads using the Californian bearing ratio of the subgrade in combination with the predicted number of standard axles using the road within the design life. Figure 23.41 shows a typical design chart from Road Note 31 . l87 Care should be taken in design, since not only is the percentage of heavy vehicles likely to be very high in a developing country but overloading is common practice. Careful traffic forecasting, including field measurement of actual axle loads, is essential to a realistic design. If it is desirable to provide, at the time of construction, a pavement capable of carrying more than 500 000 standard axles, the designer may choose either a 150mm base with a 50mm bituminous surfacing or a 200 mm base with a double surface dressing. For both of these alternatives, the recommended subbase thickness is indicated by the broken line. Alternatively, a base 150mm thick with a double surface dressing may be laid initially and the thickness increased when 500000 standard axles have been carried. The extra thickness may consist of 50 mm of bituminous surfacing or at least 75 mm of crushed stone with a double surface dressing. The largest aggregate size in the crushed stone must not exceed 19 mm and the old surface must be prepared by scarifying to a depth of 50 mm. For this

Surfacing dressing

150 mm of base

stage construction procedure, the recommended thickness of sub-base is indicated by the solid line. Table 23.20 shows how rapidly the damaging power increases with increasing axle load. 23.14.5 Structures Complex steel or reinforced concrete structures are generally inappropriate for developing countries since their construction will often involve imported materials and resultant high cost. Also the necessary expertise in construction techniques and quality control may not be readily available. Expertise in maintenance may also not be readily available and all structures should be designed with the ease of maintenance in mind. The use of pontoon bridges, drifts or Irish bridges on lowtrafficked routes may be appropriate. If a steel or reinforced concrete structure is unavoidable the designer should bear in mind the limitations of the contractor undertaking the work and the design should be kept simple. Again, many countries will have their own locally developed bridging techniques which suit local conditions; hand-dug caisson foundations, for instance, are used widely in Asia.

23.14.6 Maintenance185 I89 I9° Where low-cost construction is used the road pavement is particularly susceptible to rapid deterioration and good maintenance is vital to prolong the life of the roads. Many maintenance activities must be carried out at frequent intervals. Many loan agencies now put great emphasis on maintenance,

Either 50 mm of bituminous surfacing and 150 mm of base or

200 mm of base with surface dressing

Thickness of sub-base and/or selected fill (mm)

Minimum thickness of sub-base of 100 mm to be used with subgrades of CBR 8 to 24%. Material used in this zone to have CBR of not less than 25%

Cumulative number of standard axles in one direction (x 106) Figure 23.41 Pavement design chart for flexible pavements. (After Transport and Road Research Laboratory (1977) A guide to the structural design of bitumen-surfaced roads in tropical and sub-tropical countries. HMSO, London)

Table 23.20 Factor for converting numbers of axles to the eqivalent number of standard 8200 kg (18 000 Ib) axles

7

Axle load (kg)

8

(Ib)

Equivalence factor

910 1810 2720 3630 4540 5440 6350 7260 8160 9970 9980 10890 11790 12700 13610 14520 15430 16320 17230 18140 19070 19980 20880 21790

2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 36000 38000 40000 42000 44000 46000 48000

0.0002 0.00025 0.01 0.04 0.08 0.2 0.3 0.6 1.0 1.6 2.4 3.6 5.2 7.2 9.9 13.3 17.6 22.9 29.4 37.3 47 58 72 87

9 10 11

12

13

14 15 16 17 18

19 20 21 22

since many low-cost roads built with financial aid have deteriorated prematurely because of the lack of maintenance. Many projects now include the setting-up and running of an ongoing maintenance management system as an integral part of the project. Since the cost to the road user rises significantly as the standard of the pavement surface deteriorates, the economic advantage of maintenance is very marked. This is particularly true of low-cost roads with light construction where lack of maintenance can lead to the rapid development of corrugations, ruts and potholes.

Acknowledgements

23 24 25 26 27 28 29

The author acknowledges the patient help and assistance of his colleagues in the Croydon and Winchester offices of Mott, Hay & Anderson in the preparation of this chapter, and in particular H. Williams, J. Prince and Mrs P. Corston.

31

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