We have all had the experience of driving on a major highway and running into stop and go traffic, the first thought you have is that there must be an accident just over the hill or around the bend. But when you get to the top of the hill or around the bend the traffic starts to flow and there is no accident – what caused the slowdown. Two things traffic volume and “line of sight.”
The Line of Sight or Sight Distance is defined as the length of road surface a driver can see and have an acceptable reaction time. The people that are responsible for designing our highways, the American Association of State Highway and Transportation Officials (AASHTO), have guidelines concerning “line of sight,” and from those guidelines developed the all-important Decision Sight Distance (DSD)
AASHTO defines DSD as the distance needed to recognize a problem and complete a maneuver safely and efficiently. And according to the scientists who have done an enormous amount of research on driver reaction time the “average” driver needs 2.5 seconds to complete the “recognize a problem” part of the DSD.
The questions then become – How much distance do you use up before you get to the “maneuver safely and efficiently” part of the process. A good rule of thumb is that for every 10 MPH a driver needs 40 Feet of sight distance. If you are moving at a highway speed of 60MPHyou would need 240 Feet (40X6) of sight distance at 75 MPH you would need 300 Feet. Hence if you are driving on a highway and for whatever reason, usually a hill, or a series of bends in the road, and there is not 240 to 300 feet of sight distance, drivers will slow down. If the volume of traffic is sufficient, it will cause an accordion effect. Please keep in mind that these distances are for the average driver
Sight distance plays a major role in supplying safe and secure transportation. It is a major factor in determining if the event you drive into is winnable. During your Route Survey, know how far and never drive faster than you can see. Which means never drive at a speed that will not give you the time to react at the given sight distance.
As you are conducting a Route Survey, the question you need to ask yourself is “ At the speed I am moving, with the given sight distance, how much time do I have, and in that time frame what can I do with the vehicle. It makes no difference what training you received and where you received it or what type of vehicle you are driving. No matter what the scenario, accident or vehicle violence, if you don’t have enough sight distance at the speed you are moving it is a no win situation.