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. However, 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 distance is defined as the length of road surface a driver can see and have an adequate 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. Also, 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 question then becomes – 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 60 MPH, you 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 vital role in supplying safe and secure transportation. It is a significant factor in determining if the event you drive into is winnable. During your route survey know how far you can see and DO NOT DRIVE FASTER THAN YOU CAN SEE – Which means drive at a speed that will 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 I can do with this vehicle? 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 scenario. Your training must take this into account.
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