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 vast 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 received the citation?
Most drivers would answer the first question, “Yes”. A yes answer to the second question applies to far too many.
These questions 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 up to 20mph did not reduce the speed of traffic and raising speed limits up to 15mph did not increase the speed of traffic. Many other studies have been completed since with similar results. Click here to read the study.
An engineering study performed downstream of a location where two officers were issuing speeding tickets indicated no change in the speed of traffic due to the enforcement activity. This would confirm the conclusion of many drivers that the speed of traffic may slow temporarily, but soon resumes normal speed after passing enforcement officers.
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 is a regrettable myth. Nonetheless, the speed limit can serve an important purpose when its use is restricted 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.
Traffic engineers have determined that a speed limit meeting these requirements is near the upper 85th percentile speed of vehicles in normal traffic flow, which means that 85% of drivers would comply with speed limits set at this value and only 15% would exceed them.
A boon to all motorists would be a simple rule that would identify the safest speed to travel in any stream of traffic.
A study published in 1963 by David Solomon, and later confirmed by others, identified 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) decreases rapidly, falling to a minimum approximately at the 85th percentile, which is a speed slightly faster than the average speed of traffic. To find this “sweet spot” drivers would pass eight or nine vehicles (except for trucks and other slow 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.
LEGAL OR SAFE?
It is
far better to be passing vehicles at a moderate speed where a threatening
maneuver can easily be noted and avoided than to be surprised by a
collision with an unnoticed vehicle approaching from the rear.
Motorists should recognize that vehicles approaching from behind are "guided missiles" and hope that none have their cross-hairs lined up on them. The slower a driver 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 always 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 less fair and more stressful than it should be. Click Here to Return to Home Page.