Factors controlling geometric design

Geometric design fixes the visible dimensions of a highway - cross-section, horizontal and vertical alignment, sight distance and intersections - so that traffic is safe, comfortable and economical. The following factors control it.

1. Design speed

The most important control. It depends on road class, terrain and traffic. All elements (curve radius, sight distance, superelevation, gradient) are linked to it. NRS 2070 gives design speed by road class and terrain.

2. Topography (terrain)

Plain, rolling, hilly (mountainous) and steep terrain are classified by cross-slope. In steep terrain, speed and standards are lower.

3. Traffic factors

• Volume (AADT, design hour volume such as the 30th hour) decides the number of lanes and widths.
• Composition (cars, buses, trucks, non-motorised vehicles) and the capacity in PCU.
• Future traffic growth over the design life.

4. Design vehicle

Dimensions and turning radius of the largest vehicle (width, length, wheelbase, height) fix lane width, radius, extra widening and clearance. Vehicle performance (acceleration, braking) decides gradient and sight distance.

5. Human factors

Driver's reaction time (PIEV), eye height, visual ability, behaviour and pedestrian characteristics.

6. Environmental and aesthetic factors

Noise, pollution, scenery, land use; harmony of the road with the landscape.

7. Economy

Cost of construction, land, maintenance; benefit-cost basis. Standards must be balanced against funds.

8. Safety and capacity

Geometry should minimise accidents, provide sufficient capacity and level of service.

9. Road classification

National highway, feeder road, district, urban and village road have different standards (NRS 2070).

Importance of geometric design

• Safe, smooth and efficient movement; lower accident rate.
• Lower vehicle operating cost and travel time.
• Proper use of land and funds, with room for future improvement.