Cross Sectional Elements of Road, Roadway Design
Cross-Sectional Elements of Road
- Pavement surface characteristics
- Width of carriageway
- Road margins
- Width of formation
Pavement surface characteristics-
- Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed.
- The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as f.
- IRC suggests the coefficient of longitudinal friction as 0.35-0.4 depending on the speed and the coefficient of lateral friction as 0.15
- Even if a road is constructed with high-quality pavers, it is possible to develop unevenness due to pavement failures.
- Unevenness affects the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres.
- Bump Integrator is a device used to measure unevenness.
- LIGHT REFLECTION
- White roads have good visibility at night but caused glare during day time.
- Black roads have no glare during the day but have poor visibility at night.
- Concrete roads have better visibility and less glare.
- The pavement surface should be absolutely impermeable to prevent the seepage of water into the pavement layers.
- Further, both the geometry and texture of the pavement surface should help in draining out the water from the surface in less time.
- Camber or cant is the cross slope provided to raise the middle of the road surface in the transverse direction to drain of rainwater from the road surface.
- Camber is measured in 1 in n or n%
- Kerbs indicate the boundary between the carriageway and the shoulder or islands or footpaths.
WIDTH OF CARRIAGEWAY
- The width of the carriageway or the width of the pavement depends on the width of the traffic lane and the number of lanes. The width of a traffic lane depends on the width of the vehicle and the clearance.
- Side clearance improves operating speed and safety. The maximum permissible width of a vehicle is 2.44.
The portion of the road beyond the carriageway (pavement) and on the roadway can be generally called road margin.
- Shoulders are provided along the road edge and are intended for accommodation of stopped vehicles, serve as an emergency lane for vehicles and provide lateral support for base and surface courses.
- The shoulder should be strong enough to bear the weight of a fully loaded truck even in wet conditions. It is desirable to have a width of 4.6 m for the shoulders.
- Parking lanes
- Parking lanes are provided in urban lanes for side parking. Parallel parking is preferred because it is safe for the vehicles moving on the road.
- The parking lane should have a minimum of 3.0 m width in the case of parallel parking.
- Bus bays are provided by recessing the kerbs for bus stops. They are provided so that they do not obstruct the movement of vehicles in the carriageway.
- They should be at least 75 meters away from the intersection so that the traffic near the intersections is not affected by the bus bay.
- Service roads
- Service roads or frontage roads give access to access controlled highways like freeways and expressways. They run parallel to the highway and will be usually isolated by a separator and access to the highway will be provided only at selected points.
- These roads are provided to avoid congestion in the expressways and also the speed of the traffic in those lanes is not reduced.
- Cycle track
- Cycle tracks are provided in urban areas when the volume of cycle traffic is high Minimum width of 2 meters is required, which may be increased by 1 meter for every additional track.
- Footpaths are the exclusive right of way for pedestrians, especially in urban areas. They are provided for the safety of pedestrians when both pedestrian traffic and vehicular traffic are high.
- The minimum width is 1.5 meters and may be increased based on the traffic. The footpath should be either as smooth as the pavement or smoother than that to induce the pedestrian to use the footpath.
- Guard rails
- They are provided at the edge of the shoulder usually when the road is on an embankment. They serve to prevent the vehicles from running off the embankment, especially when the height of the fill exceeds 3 m.
- Various designs of guard rails are there. Guard stones painted in alternate black and white are usually used. They also give better visibility of curves at night under the headlights of vehicles
- Width of formation
- Width of formation or roadway width is the sum of the widths of pavements or carriageways including separators.
- and shoulders. This does not include the extra land in formation/cutting.
Right of way (ROW)
- Right of way (ROW) or land width is the width of land acquired for the road, along its alignment. It should be adequate to accommodate all the cross-sectional elements of the highway and may reasonably provide for future development.
- To prevent ribbon development along highways, control lines and building lines may be provided.
- A Control line is a line which represents the nearest limits of future uncontrolled building activity in relation to a road.
- A building line represents a line on either side of the road, between which and the road no building activity is permitted at all.
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