By John Carr, NMA Massachusetts Activist
“The [accident] involvement rate was minimum at a speed that was usually 5 or 10 miles per hour more than the average speed on the study sections.” — David Solomon (noted transportation researcher)
You may have heard of the “85th percentile rule” for setting speed limits. It is variously treated as an object of religious devotion or hatred by people who don’t understand the reasons behind it.
The rule in its most literal form is a mistake, but the principle is important.
These are some factors, based on past experience in the United States, that support the rule. They are not necessarily true in all countries for all time.
• Observation that vehicles at or slightly above the average speed of traffic crash least often.
• Observation that the average speed of traffic is not affected by speed limit signs.
• Observation that drivers generally agree on a speed for a road, most staying within ±5 mph of the average, while policy makers’ subjective opinions disagree wildly
• Opinion of police and accident reconstruction experts that most crashes could not have been prevented by driving slower.
• Speculation that smoothness of traffic flow (but not average speed) is affected by speed limit signs.
• Democratic principles. If we ever did have to trade off safety and speed, the opinion of the majority ought to decide the best compromise.
Ever heard “it’s a limit, not a target”? If speed limits are limits they need to be above whatever speed is safest, not right at that speed. The average driver should not be 1 mph away from going to jail.
The speed limit needs to be at least 5 mph above the average speed of traffic. That is the absolute minimum that can be justified by the above principles.
Regarding the 85th percentile, treat it as a synonym for “5 mph over the average speed.” Literally it means 85 percent of drivers are going slower and 15 percent are going faster.
A famous study found drivers near the 85th percentile speed crashed least often. This speed-crash relation is called the “Solomon curve.” People said use that as the speed limit, but that’s a mistake because the safest speed should be clearly legal.
The safest percentile seems to vary by ±5 mph from the 85th percentile depending on type of road. The right limit might be 5 over the average in a business district but 15 over on an Interstate. That’s another reason the strict form of the “85th percentile rule” is a mistake.
Most agencies pretend to consider traffic speed but violate the above principles. They use 5 over the average as the maximum allowable speed limit, and then only if nobody is likely to complain. Any halfway competent public relations engineer can make up an excuse to go slower. FHWA has endorsed an “expert system” called USLIMITS2. It provides a toolkit of excuses, based on unsubstantiated opinions, for unimaginative officials to use.
Thanks to an endless supply of excuses, the typical speed zone in America is below the average speed of traffic.
In a way these policies are worse than a made up limit. At least the arbitrary limit might be right occasionally. Most agencies would rather always be wrong.
