The Layout and Construction of Roath T HE 30-page memorandum on
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the layout and construction of roads recently issued by the Ministry of Transport was prepared in consultation with. the Institution of Municipal and County Engineers and the County Surveyors Society, and is, in effect, a code for road builders, but, like the Highway Code, it is in the nature of advice rather than an instruction, and the notes given will be revised from time to time as required. It also points out that the recommendations may not be capable of immediate or universal application. Populous areas may require treatment different from that appropriate in rural areas, and whilst there is scope for much greater standardization of practice, a rigid uniformity is not always desirable or possible of attainment.
Factors to Investigate.
A necessary preliminary to the improvement or diversion of an existing road is an investigation of traffic conditions, the volume and direction of the main flow, points of congestion, the incidence of standing vehicles and development and industrial growth at terminal points. Any restriction placed upon traffic using a particular road is likely to influence traffic conditions on other roads in the vicinity.
Schemes for controlling traffic in any particular area should be planned to utilize, appropriately the advantages of roundabouts, refuges, warning and erection signs, traffic-control signals, etc., and consideration should be given to the modification of existing layouts.
Much can be done to reduce the risk of accident by the improvement of the highway system, adapting its ,Ateuelral layout to meet the changes in traffic conditions, limiting the number and controlling the design of points of access to important roads, and withholding consent to building on frontage land, except on conditions to secure that adequate provision may be made off the highway to accommodate tradesmen's vehicles and those of others calling at such buildings.
Preservation of Trees.
Trees should not be felled unless this course be unavoidable. They can he left standing on grass verges, and the roadway' deviated to secure their preservation.
• In rural districts, it is undesirable that the space immediately adjoining the road should be used as a footpath. Judicious planting of shrubs will obviate this risk. At bus stops the footpaths should be extended across the verge.
Layout depends primarily upon the volume and character of the potential traffic. As we mentioned last week, the unit width for each lane should be normally 10 ft., and new carriageways should have a minimum width 'of 20 ft., except in sparsely populated area where the traffic is light. If there be a large proportion of Wide vehicles, the unit width should be increased to 11 ft. When improving streets in urban areas it will not always be possible to provide a system of dual carriageways. The segregation of certain classes of traffic is of the utmost value in the promotion of safety, and in determining the stytidard width provision should be made for such future widening as may be rendered necessary by the construction of cycle tracks and footpaths.
Roads likely to be used by a large number of 'heavy goods and publicservice vehicles will repine adimension of 66 ft. between the outer kerbs of dual carriageways to proyide adequate turning space. In such cases, dual carriageways, each with two traffic' lanes, with footpaths, but not cycle tracks, will have an overall width of 80 ft., and those with cycle tracks in addition, 120 ft., compared with the 8(1 ft. and WO ft. respectively, proposed in Circular 454 (Roads).
What Determines Traffic Capacity.
The traffic capacity of a road is largely influenced by its road junctions and intersections. The latter should be so arranged as to minimize interference with the flow.
In improving existing roads, long curves or straight lengths should be used to eliminate frequent changes of direction. The curves should have the largest radius practicable. 1,000 ft. being regarded as a minimum radius. If a smaller radius than 500 ft. must be adopted, the width of the carriageway for erielt traffic lane should he increased by 2 ft. For a curve of 1,500 ft. to the smaller figure, an allowance of 12 ins. should be made; this should usually be on the inside of the curve. The width should be brought back to the normal at each end by an easy transition.
Super-elevation should be applied where practicable on all curves. In built-up areas this is governed by varying factors, and in each case must be treated individually.
Normal gradients of 1 in 30 may be regarded as the maximum.' The requirements of hilly districts may call for the introduction of steeper gradients, or it may be necessary in order to avoid excessive filling in' or excavation. To facilitate surface drainage the channels of the carriageway should provide's:minimum fall of 1 in 250.
Visibility is important at all changes of gradient and at bridges. Driver should have a clear view of any vehich proceeding in the opposite directior when it is not less than 500 ft. iron them, at an eye level of 3 ft. 9 ins above the carriageway. Where this impracticable the traffic should be con trolled by the marking of lanes. Thi! applies both to vertical and horizonta curves.
In urban areas splayed corners art preferable to semi-circular corners. Or important routes in these areas sub sidiary roads should enter at righi angles and the splay should be of 4: degrees, commencing not less that 30 it. from the intersection of the frontage lines produced.
Fencing with vertical bars on tin inner side of sharp curves should in avoided; they obstruct the vision o. drivers, Reflectors That Are Helpful.
Red and white reflectors on blact and white posts indicating the near m off side of the earriagewa,y, respectively, should be helpful. They are aisc suitable for telegraph poles, bridge parapets, kerbs, at Y junctions, etc.
Lines to form traffic lanes shoulc give a width of not less than 10 ft. They need not be continuous, but may consist of sections 3,ft. long and 4 ins, to 5 ins, wide interrupted by gaps o1 15 ft. maximum. It is undesirable tc provide three traffic lanes on curves ol leas than 2,000-ft. radius. .
. In designing junctions on new roads they should be spaced not less than 44C yards apart. Major traffic route; should be rendered independent of local roads, by being carried over or under them. Junctions in excess of four ways should be avoided, except where there is ample space for a suitable roundabout.
Merits of the Roundabout.
A roundabout has the advantage of dealing with a greater volume of traffic than where traffic is controlled en the stop-and-go principle, but where a major road is intersected by anothei carrying sufficient traffic to require control, and the pedestrian traffic is likely to become considerable, preference should be given to traffic signals. They may be installed in the first instance, but where possible, land should be acquired to provide later for a roundabout. Where traffic at a roundabout is at times continuous, consideration should be given to pedestrian subways or bridges.
To screen headlights the surface el a central island should be sloped to a height of 4 ft. at the centre, or planted with shrubs.
Where there is heavy pedestrian traffic, guard rails are advisable adjacent to the kerbs, with -openings al pedestrian crossings. In rural surroundings hedges maintained at a height not itot exceeding 3 ft. may be employed for this purpose.