Question

Problem 1 (Concepts of safe distances): In lecture 7 and 8, we discussed five (5) types of safe distances, such as braking distance, deceleration distance, stopping sight distance, decision sight distance, and passing sight distance. Explain what they are in terms of definition, situation of concern, and possible applications for analysis, design and management (at least one example for each

Answer 1

Types of safe distance:

1. Braking Distance: Braking distance is calculated as the distance the vehicle travels after the brakes being fully applied. It is affected by the vehicular speed, friction between the tires and the road and the air drag. As far as highway safety is concerned, braking distance is of prime importance. The situation of concern is when the braking distance is more which can be due to the speed, types of brakes, friction, type of tires etc. Braking distance is used for the calculation of stopping sight distance. It is very difficult to achieve reliable calculations of the braking distance as road conditions and the tires grip can vary greatly. For example the braking distance may be 10 times longer when there is ice on the road.

2. Deceleration Distance: The additional distance a vehicle travels from the point the deceleration starts to the point the vehicle stops is known as deceleration distances. It is similar to braking distance. If the deceleration distance is more, it is a situation of more concern. It mostly depends on the coefficient of friction. Deceleration distance is necessary to calculate the stopping sight distance and to analyse the safety conditions of the roads. For example, vehicles with more deceleration distance will take more time to stop.

3. Stopping Sight Distance: It is a sight distance used in road design. It basically is the worst case distance a driver needs to be able to see in order to have distance to stop before colliding with another vehicle or object. If the distance between the vehicle and the object is less than that of the stopping sight distance, the vehicle will collide with the object. Many issues persist concerning the development , presentation, adequacy, and usability of the AASHTO design values for the stopping sight distance. It is the sum of the reaction distance and the braking distance. Stopping sight distance is a critical factor in designing the safety of the highway and improper calculations can lead to a collision. For example, if a driver realises lately that he might collide, the distance available to stop will be less than the stopping sight distance. Higher speeds in congested roads can lead to such situations.

4. Decision Sight Distance: It is defined as the distance at which the driver can detect a hazard or a signal in a roadway environment, recognize it or its potential hazard, select an appropriate path and perform the required action safely and efficiently. Poor decisions can lead to hazards and collisions. It must be analysed in such a way that there will be ample amount of distance available to stop the vehicle. Research was done to relate this concept to specific road types, design speeds, traffic operating and driver attributes. Primary factors are perception reaction time, driver eye height, objective height, vehicle speed, friction, deceleration rates and roadway grade. The design of the roadway should be such that there will be safe reaction distance available to an average driver. Foe example, a driver must be able to decide normally and then stop its vehicle before any collision.

5. Passing Sight Distance: It is the minimum sight distance that is required on a highway, generally two lane, two directional one, that will allow a driver to pass another vehicle without colliding with a vehicle in the opposing lane. It is a critical component for the design of two-lane highways. AASTHO defines Passing sight distance as having three main distance components - Distance traveled during reaction time and acceleration into the opposing lane, distance required to pass in the opposing lane and distance necessary to clear the slower vehicle. The roadway design should focus on provide ample passing sight distance so that a driver can easily pass the slower vehicle without colliding with the vehicle in the opposite direction. In case of narrower and congested roads, the passing sight distance must be carefully evaluated to maintain proper safety. For example, a driver decides to overtake a vehicle in a two-lane road and there is less passing distance, it can collide with the vehicle in opposite direction and the slower vehicle.