SPEED LIMITS AND DRIVING SAFETY
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 at twice the speed.
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 means to control this important safety factor.
HOW WELL ARE SPEED LIMITS WORKING?
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.
So, what is going on? Either motorists are blatant lawbreakers flouting the law in their own selfish interest or government is at fault for posting unreasonably low speed limits. We have a problem that needs to be solved!
Unfortunately the expectation that speed limits would be an effective tool in controlling vehicle speed is seriously flawed. Numerous carefully conducted traffic studies show little, if any, relationship between the speed of traffic and posted speed limits.
Although the vast majority exceeds posted speed limits by a significant amount, traffic flows very safely. Fatality rates are extremely low and declining in the face of rampant violation of speed limit laws. Apparently, motorists intuitively make reasonable decisions when selecting the speed at which they travel although they seldom comply with posted speed limits.
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. Click here to read the study. LIMITATIONS ON THE APPLICATION OF SPEED LIMIT SIGNS These studies show that speed limit signs do not affect the speed of traffic. In spite of popular opinion to the contrary, it turns out that the idea that speed limits are a useful tool for controlling traffic speed is an insidious myth. Nonetheless, they can serve an important purpose when their use is restricted to the following important functions: 1. Inform motorists of the speed considered to be a reasonable maximum under favorable conditions by the majority of drivers who regularly use the roadway, 2. Provide a legal basis for prosecution of those who drive dangerously fast, and 3. Notify motorists of high risk locations where hazards exist that are not immediately apparent, such as school zones. In order to use speed limits fairly, we need a method to satisfy these requirements without creating a situation where speeding citations are issued to motorists who are driving safely. A method for determining such speed limits is strikingly obvious: Observe the speeds driven by typical, safe drivers, note the maximum speed chosen by the vast majority when driving under favorable conditions, and use this speed as the primary basis for setting speed limits. Speed limits based on this criterion will be voluntarily obeyed and they will also be low enough to identify motorists who drive dangerously fast. This criterion is not new to traffic engineers. They have determined that an appropriate speed limit is near the upper 85th percentile speed of vehicles in normal traffic flow, which means that 15% of drivers would exceed speed limits set at this value. It is obvious, however, that far more than 15% drive faster than most posted speed limits, which is compelling evidence that they are set lower than this recommended value. These low speed limits create lawbreakers of safe drivers and speed traps that allow officers to issue speeding tickets to safe drivers when they are driving safely. We may ask ourselves, how can this be? What has happened to create such inequities? The answer may be that government agencies responsible for traffic laws mistakenly believe that they can control traffic speed by posting speed limits. and may mandate speed limits that are too low, unintentionally setting up speed traps that abuse the public, and create other problems. SAFEST DRIVING SPEED A boon to all motorists would be a simple rule that would identify the safest speed to travel in any stream of traffic. Of course the safest driving speed constantly changes as motorists proceed on their way. For example, the maximum safe driving speed is considerably faster in light traffic than when traffic is congested or under poor weather conditions. We have little control over these factors, but we can choose to drive more slowly under adverse conditions. As a matter of safety, the speed of traffic by itself is less important than the difference in speed between vehicles. It is an established scientific fact that vehicles moving significantly slower or faster than the average speed of traffic are at higher risk for crashing than those traveling near the average speed. 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 his study are shown in the chart below.
Many other carefully conducted studies have been completed since with similar results.
Note that the minimum crash risk (involvement rate) correlates with a speed slightly faster than the average speed of traffic. In the language of statistics, the minimum risk occurs at approximately the 85th percentile of the traffic speed distribution. That is to say: At the safest speed, a vehicle is traveling faster than 85% and slower than 15% of the vehicles in a traffic stream.
Therefore, to select the safest speed, especially in heavy traffic, drivers should choose a speed that is faster than 85% of the vehicles and slower than 15%. To find this “sweet spot” drivers would pass eight or nine vehicles for every vehicle that passes them. This is the working rule for choosing the safest speed.
Upon reflection, this makes perfect sense. The 85th percentile is usually only slightly faster (approximately 1-3 mph) than the average speed 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.
If common sense indicates otherwise, strict application of the pass eight or nine rule may not be advisable. For example, in light freeway traffic the majority of vehicles often assume safe speeds with little passing occurring. In these situations the best strategy may be to stake out a position near the mid-pint of a gap in the traffic and travel at the average speed of traffic, occasionally passing slower vehicles such as trucks.
A motorist may consider vehicles approaching from behind as guided missiles—hoping that none have their crosshairs lined up on their vehicle. The slower a driver chooses to drive, the faster the missiles approach and the greater their number.
It is not unusual for speed limits to be posted ten miles per hour or more lower than the 85th percentile. In heavy traffic, following vehicles swarm past motorists who drive at these posted speeds, placing them at a significantly higher risk of a collision.
It is far better to be passing vehicles at a moderate speed where a threatening maneuver can easily be noted and likely avoided than to be surprised by a collision of an unnoticed vehicle approaching from the rear.
Thus, paradoxically, in most situations motorists must violate speed limits by a significant amount in order to drive at the safest speed. This places them in an extremely awkward position wherein they must choose between safety and obedience. It should be no surprise that they typically choose the safer option: To drive faster than speed limits.
Before we leave this point, let’s address a question that may have occurred to the reader: If every driver should seek the 85th percentile by “passing eight or nine vehicles for each one that passes them”, wouldn’t the speed of traffic increase to dangerously high levels?
The answer is no because there is an upper comfortable and safe limit for each responsible driver that they will not exceed regardless of how fast others are driving. For most drivers, this safe limit is very close to the 85th percentile speed. In fact, if all drivers would seek the 85th percentile, safety would be improved as explained below.
When a speed limit is increased to the 85th percentile, obedient drivers who have obeyed the previous lower speed limit will be likely to approach the new speed limit while those who normally drive near the 85th percentile speed will continue to do so.
As the formerly slower drivers move toward the 85th percentile speed, variation in speed between vehicles and passing incidents decrease, improving safety.