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 are traffic volume and “line of sight.”
The line of sight distance is defined as the length of road surface drivers 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 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 or 16.1 KPH, a driver needs 40 Feet or 12.2 Meters of sight distance.
If you are moving at a highway speed of 60 MPH or 95.5 KPH, you would need 240 Feet or 73 Meters (40X6 feet or 12.2X6 meters) of sight distance. At 75 MPH or 121 KPH, you would need 300 Feet or 91.4 Meters.
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 – or 73 to 91 Meters 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.
Secure Transportation
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 can I do with this vehicle? No matter 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.