Road Width | What is the width of a road?
Road Width means the distance between the boundaries of a road including footpath and drains measured at right angles to the center of the plot.
The road width should be reduced to minimize the cost of construction and maintenance when it is sufficient to carry traffic loading efficiently and safely.
The following factors should be taken into account when choosing a road width:
1. Road classification. The road is usually classified according to its function in the road network. The higher the class of the road, the higher the level of service expected and the wider the road should be.
2. Traffic. Heavy traffic on the road is more frequent than passing vehicles and overtaking slow vehicles, so the lanes of the road are longer and the traffic lanes should be wide.
3. Vehicle Dimensions. Common steering deflections and tracking errors, especially heavy vehicles, reduce the clearance between passing vehicles. Most truck percentages require broad traffic lanes.
4. Speed of the vehicle. As speed increases, drivers will have less control over the vehicle’s lateral position, reduce clearances and therefore need wider traffic lanes.
Figure 9.1 shows typical cross-sections recommended by Overseas Road Note 6 (ref.1) for A-F for various road design classes. These design classes and related traffic
Access road width
For access roads with low volumes of traffic (<20 AADT) single-lane operation is adequate as there will be only a small probability of vehicles meeting, and the few passing manoeuvres can be undertaken at very reduced speeds using either passing places or shoulders.
Provided meeting sight distance is available, these manoeuvres can be performed without hazard, and the overall loss in efficiency brought about by the reduced speeds will be small as only a few such manoeuvres will be involved.
For higher traffic flows (20–100 AADT), single-track roads cause considerable inconvenience to traffic and it is only to be recommended for short roads or in hilly/mountainous terrain where the cost of construction in side cut and the subsequent haulage cost of materials is high.
Most countries with a rural access road program have provided roads that are sufficiently wide for two vehicles to pass safely.
It should also be noted that, if the new road is to be constructed by machine, the extra construction cost of building a wider road will in level and rolling terrain be quite small.
The Public Works Department in Bhutan, a very mountainous country, recommends single-lane pavements of 3.0–3.5 meters width for traffic volumes up to 200 AADT and with shoulder widths of 0.5–2.0 meters as shown in Table 9.1 (ref. 2).
On unpaved roads, the running surface normally extends to the full width of the formation. Where the formation materials have insufficient bearing capacity traveling should therefore be made in the full formation width.
Where it is anticipated that a paved surface will be provided in the future, consideration should be given to extending the width of the formation at the time of construction to allow for this.
Collector road width
On roads with medium volumes of traffic (100–1000 AADT), the number of passing maneuvers will increase and pavement widening will become worthwhile operationally and economically.
However, in view of the generally high cost of capital for construction in developing countries and the relatively low cost of travel time, reductions in speed when approaching vehicles pass will remain acceptable for such flow levels, and running surface widths of 5.0 and 5.5 meters are recommended by the TRL Overseas Unit.
However, in the case of a high percentage of heavy vehicles (>40%), it is advisable to increase the running surface width to 5.5 or 6.0 meters.
Arterial road width
For arterial roads with high flows (> 1000 AADT), the 6.5-meter wide running surface allows safe vehicles to pass safely in their lanes without the need to move or slow down.
The standard cross-sections proposed in the SATCC recommendations (ref. 3) are listed in Table 9.2, and the layout of cross-sections with an equipped carriageway is shown in Figure 9.2.
Culverts and bridges
The cross-section of the road is usually maintained across culverts, but special crosssections may need to be designed for bridges, taking into account traffic such as pedestrians, cyclists, etc., as well as motor traffic.
For example, a reduction in carriageway width may be acceptable when an existing narrow bridge has to be maintained because it is not economically feasible to replace or extend it.
It may also sometimes be economically constructed a superstructure of reduced width initially with provision for it to be widened later when traffic warrants it.
In such cases, proper application of traffic signs, rumble strips, or speed bumps is required to alert motorists of a roadblock.
Places should be provided to allow for crossing and overtaking of one-lane roads without shoulders. The total width of the road at crossing points should be at least 5.0 meters but preferably 5.5 meters, which allows two trucks to pass safely at low speeds.
The length of individual passes varies with local conditions and the sizes of vehicles in general use but, in general, 20 meters, including tapers, can supply trucks with a wheel base of 6.5 meters and an overall length of 11.0 meters.
Generally, the crossing points should be 300-500 meters each, depending on the terrain and geometric conditions. They must be located at a distance from each other and built in the most economical places determined by terrain and ground conditions, such as transitions to fill by cut, but at precise intervals.
The carriageway widths should be increased on low-radius curves to allow for the swept paths of longer vehicles, and the necessary tolerances in lateral location as vehicles follow a curved path. The levels of widening recommended by Overseas Road Note 6 for single-lane and two-lane roads are shown in Tables 9.3 and 9.4 respectively.
The lane widening in curves (widening of each lane) according to the SATCC Recommendations are shown in Table 9.5.
Widening should be applied on the inside of a curve and be gradually introduced over the length of the transition curve or the superelevation development length. On access roads, which often have substantial horizontal curvature requiring local widening, it may be practical to increase width over a complete section to offer a more consistent aspect to the driver.
This enhancement of the standards should be undertaken where other advantages such as easier construction or maintenance can be identified and where the additional costs are acceptably small.
Shoulders provide for the accommodation of stopped vehicles. Properly designed shoulders also provide an emergency outlet for motorists finding themselves on a collision course and they also serve to provide lateral support for the carriageway.
Further, shoulders improve sight distances and induce a sense of ‘openness’ which improves capacity and encourages uniformity of speed.
In developing countries shoulders are used extensively by non-motorized traffic (pedestrians, bicycles and animals) and a significant proportion of the goods may be transported by such non-motorized means.
Shoulders are recommended on all classes of roads except minor access roads in hilly/mountainous terrain where shoulders would constitute a considerable additional cost.
For access roads, the combined width of the carriageway and shoulders is recommended to be 6.0 meters, which is sufficient for two trucks to pass with 1.0-meter clearance.
A minimum of 1.0 meters of surfaced shoulders is recommended for the paved collector and arterial roads to avoid damage and the resulting break up of the edge of the carriageway pavement.
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For roads with high traffic flow, 2.5-meter-wide shoulders should be provided as stopped vehicles blocking any part of the carriageway would cause a significant hazard. Of the 2.5-meter shoulder, at least one meter should be paved, for instance by a double seal surface treatment.
Paved shoulders shall be clearly segregated from the carriageway either by use of 100mm-wide carriageway edge line marking or by sealing the shoulders with a different colored aggregate.
In the case of selecting road width, we should know the important factors which we have to consider. We have also seen about the road width of different classified roads with references of IS standard codes.
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