Geometric Design of Highway

Geometry of highway should be designed to provide efficient and comfortable operation of traffic. Layout  design of road section joining two roads with different direction is referred to as geometric design of horizontal curves and the joining of two roads with different gradients is referred to as geometric design of vertical curve. 

Factors Controlling Geometric Design :

• Topography or Terrain
• Design Speed
• Vehicle characteristics 
• Traffic
• Traffic Capacity
• Road user characteristics 
• Environmental consideration
• Economy in construction, Maintenance and operation vehicles  

Topography or Terrain:

The topography of terrain of the area from which the road is passing is also controls the geometric design of highway. Cross slope of highway is totally depends on the type of topography or terrain of that particular area.

Type of Terrain Topography	Plain	Rolling	Hilly	Steep
Cross slope (in % )	0-10	11-25	26 - 60	> 61

Design Speed :

Design speed is the maximum speed at which vehicles can continuously run safely under the favorable conditions. The design speed definitely has to be correlated with the terrain conditions and the classification of the highways like NH, SH, MDR, ODR, VR etc. 

Different design speed for different roads / highways as per IRC,

Classification of Road	Plain Terrain		Rolling Terrain		Hilly terrain		Steep Terrain
	Ruling	Minimum	Ruling	Minimum	Ruling	Minimum	Ruling	Minimum
NH & ST	100	80	80	65	50	40	40	30
MDR	80	65	65	50	40	30	30	20
ODR	65	50	50	40	30	25	25	20
VR	50	40	40	35	25	20	25	20

In above table, suggested design speed in India for Rural Highways (kmph)

Classification of Road	Arterial Road	Sub-arterial Road	Collector Road	Local Road
Design Speed	80	60	50	30

In above table, suggested design speed in India for Urban Roads (kmph)