Geometric Design Speed of Highways

Design Speed

The design speed, as noted earlier, is the single most important f actor in the design of horizontal alignment. The design speed also depends on the type of the ro ad. For e.g, the design speed expected from a National h ighway will be much higher than a village r oad, and hence the curve geometry will vary significantly.

The design speed also de pends on the type of terrain. A plain terrain can afford to have any geometry, but for the same standard in a hilly terrain requires substantial cutt ing and filling implying exorbitant costs as well as safety concern due to unstable slopes. Therefo re, the design speed is normally reduced for terrains with steep slopes.

For  instance,  Indian Roa d  Congress  (IRC)  has  classified the terrai ns  into  four categories, namely plain, rolling, mountaino us, and steep based on the cross slope as given in table. Based on the type of road and type of terrain the design speed varies. The IRC has suggested desirable or ruling speed as well as minimum suggested design speed and is tabulated in table .

Table : Terrain classification

Terrain classification   Cross slope (%)

Plain          0-10

Rolling      10-25

Mounta inous     25-60

Steep         60

The recommended design speed is given in Table .

Table : Design speed in as per IRC (ruling and minimum)

Type          Plain  Rolling        Hilly  Steep

NS&SH     100-8 0       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

1 Topography:

The  next  important  factor  that  affects  the  geometric  design  is  the  topography.  It  is

easier to construct roads with required standards for a plain terrain. However, for a given design speed, the construction cost increases multiform with the gradient and the terrain. Therefore, geometric design standards are different for different terrain to keep the cost of construction and time of construction under control. This is characterized by sharper curves and steeper gradients.

2 Other factors :

In addition to design speed and topography, there are various other factors that affect the geometric design and they are brie y discussed below:

Vehicle: The dimensions, weight of the axle and operating characteristics of a vehicle influence the design aspects such as width of the pavement, radii of the curve, clearances, parking geometrics etc. A design vehicle which has standard weight, dimensions and operating characteristics are used to establish highway design controls to accommodate vehicles of a designated type.

Human: The important human factors that influence geometric design are the physical, mental and psychological characteristics of the driver and pedestrians like the reaction time.

Traffic: It will be uneconomical to design the road for peak traffic flow. Therefore a reasonable value of traffic volume is selected as the design hourly volume which is determined from the various traffic data collected. The geometric design is thus based on this design volume, capacity etc.

Environmental: Factors like air pollution, noise pollution etc. should be given due consideration in the geometric design of roads.

Economy: The design adopted should be economical as far as possible. It should match with the funds allotted for capital cost and maintenance cost.

Others: Geometric design should be such that the aesthetics of the region is not affected.