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 Geometric Design: Theory and Concepts

 
Stopping Sight Distance

(Note: If you feel uncomfortable with your understanding of brake reaction time or braking distance, you might want to review those topics before continuing with Stopping Sight Distance.)

The stopping sight distance is the sum of the braking distance and the distance traversed during the brake reaction time. In other words, it is the length of roadway that should be visible ahead of you, in order to ensure that you will be able to stop if there is an object in your path. 

For example, let us say that you are negotiating a horizontal curve in a highway when you notice an object 200 feet ahead of you. If the distance you travel during your brake reaction time is 100 feet and your braking distance is 130 feet, you will not be able to avoid the collision. If the horizontal curve were not as tight, you would be able to see the object at a distance of 250 feet, which would allow you to stop 20 feet short of the object. A properly designed roadway will provide the minimum stopping sight distance at every point along its length.

In order to calculate the actual sight distance based on the geometry of the roadway, some assumptions are necessary. The main assumptions are the height of the driver's eyes above the roadway surface and the height of the object or hazard. In geometric design, these values are 3.5 feet and 0.5 feet, respectively. This represents a reasonable worst-case scenario. 

To include the stopping sight distance in your design, calculate the stopping sight distance for a vehicle traveling on your roadway at the design speed, and then make sure the actual sight distance that you provide is at least as great as the stopping sight distance. 

Trucks and Busses

Trucks and busses require longer braking distances than passenger cars, but their stopping sight distances are not considered in most designs. This is because the driver's eyes are higher and their sight distance is consequently increased. The drivers of these vehicles also tend to be more experienced and more alert. The net effect is that large vehicles can avoid obstacles even though the road was not specifically designed with them in mind. The engineer must decide when large vehicles may need extra sight distances and provide these distances where necessary.