SAFE SPEED AND THE LAWS OF PHYSICS
The scientific fact that kinetic energy increases twice as fast as speed cannot be denied. When motorists double their speed, they have four times as much energy to deal with and crashes are four times more severe.
Therefore, the impact of vehicle speed on traffic safety is of utmost importance. To assure safety on our highways, vehicle speed must be carefully considered by highway designers and traffic managers and wisely controlled by motorists.
Government agencies throughout the United States have adopted the speed limit as the primary means to control this important safety factor.
Are you among the great majority of motorists who customarily drive faster than posted speed limits?
Have you ever paid a fine for exceeding a posted speed limit although you were driving safely when you were observed by the enforcement officer?
Most drivers would answer the first question, “Yes”. A yes answer to the second question may apply to far too many.
These questions may point to an unacceptable state of affairs: Nearly all motorists traveling faster than posted speed limits are driving safely and most speeding tickets are issued to drivers who were driving safely when observed by an enforcement officer.
This nonchalance toward speed limits was reported in a comprehensive study completed in 1992 by the Federal Highway Administration. This study determined that lowering speed limits as much as 20mph did not reduce the speed of traffic and raising speed limits as much as 15mph did not increase the speed of traffic. The study concludes that speed limits have little, if any, effect on the speed of traffic flowing past speed limit signs. Many other studies have since produced similar results. Click here to read the study.
An engineering study performed downstream of a location where two officers were monitoring traffic and pursuing speeders indicated no change in the speed of traffic due to the enforcement activity. Drivers continued at the speed they considered reasonable. Any governing agency that may question whether or not this result applies in their jurisdiction should conduct their own study under direction of their traffic engineer.
If it is verified that traditional speed limit enforcement has no significant effect on the speed of traffic in their jurisdiction, they should consider redirecting their resources to other traffic safety matters that would be more effective, such as enforcing coordinating laws.
LIMITATIONS ON THE APPLICATION OF SPEED LIMIT SIGNS
Unfortunately, it turns out that the idea that speed limits are a useful tool for controlling traffic speed may be a regrettable myth. Nonetheless, the speed limit can serve an important purpose when its use is limited to the following important functions:
- Provide a legal basis for prosecution of those who drive dangerously fast,
- Notify motorists of locations where hazards exist that are not immediately apparent, such as school zones, and
- Inform motorists of the safest speed to travel in normal traffic under favorable conditions.
The scientific method for setting speed limits that achieve these results is described on the page titled, "Setting Fair, Legal, Speed Limits".
A boon to all motorists would be a simple rule that identifies
the safest speed to travel in any stream of traffic. Such a rule may be based on a 1963 study by David Solomon that reveals the speed at which the probability of a crash is smallest.
The results of Solomon's study are shown in the chart below.
Note from the chart that the crash risk (involvement rate) falls to a minimum as a a vehicle approaches a speed slightly faster than the average speed of traffic indicating that the safest speed is approximately 5 miles per hour faster than the average speed of traffic. To find this “sweet spot”, excepting trucks and other slow vehicles, drivers should pass eight or nine vehicles for every vehicle that passes them. This is a good working rule for choosing the safest speed.
Upon reflection, this makes perfect sense. This speed is only slightly faster than the average speed of traffic and drivers at this speed can see virtually all problems that could affect them because they are in the field of view ahead. Any problems that occur behind them are of no concern because they are moving away from them.
Motorists should recognize that vehicles approaching from behind are "guided missiles" and hope none have their cross-hairs lined up on them. The slower a motorist chooses to drive, the faster the missiles approach and the greater their number—and the greater the chance for an unexpected collision.
The practice of cruising in the left-hand blind spot is common and dangerous. Motorists should cruise far enough behind others so that they are visible in leading drivers’ rear-view mirrors or pass promptly and cruise in plain view of following vehicles. Following is a suggestion that may help you to be aware of drivers cruising in your left blind spot.
Try adjusting the left-hand mirror outward far enough so that you can see the front of an approaching vehicle just before it leaves the view of your inside rear-view mirror. With the mirror adjusted in this manner, you may also be able to see the front of a passing vehicle in your peripheral vision before it disappears from the mirror. Except for small vehicles such as motorcycles, this may make it possible to be aware of a vehicle in your left blind spot while keeping your eyes focused ahead.
If a motorist insists on pacing you in in your blind spot, you should consider putting some distance between you and them by either slowing down or speeding up to allow a safe distance between your vehicles.
The next page discusses how well typical drivers manage their vehicles in the face of obstacles that make driving more stressful and dangerous than it should be. Click Here to Return to Home Page.