Speed Limits

Have you ever watched cars go by your home or business and felt that they were going too fast and perhaps the speed limit was too high? Complaints regarding the speed of traffic and even petitions for lower speeds are very common. Ever wonder just how speed limits are determined? Would you believe that, in some cases, it is based on whatever the majority of the existing traffic is traveling? That's right, the speed limits are frequently set following what is called the "85th percentile rule." This rule, which is used nationwide, dictates that whatever speed 85 percent of the traffic is traveling is the appropriate speed for the location in question. 

Speed Limit Brochure
 

Traffic Speed Assumptions


The theory is that most drivers are sensible and will accurately judge on their own the proper speed for the road in question. They will slow down where there are curves and hills or other factors that might affect vehicle control or sight distance, and will go faster where the road is straight and level with no sight obstructions. The 85th percentile rule is based on the following: It is generally agreed that with no traffic controls, the driver would adopt a reasonable speed for the prevailing conditions. Further, it is sometimes assumed that a certain percentage (usually 15 percent) of drivers will normally exceed a safe and reasonable speed.

To get an enforceable speed limit set or changed on a main road, it is necessary that the state police conduct a speed study and that the state police and the Road Commission concur on the speed limit. Unless the state police concur with the proposed speed limit, it is not legally enforceable. Before you run out and request a speed study be conducted to reduce the speed on your road, you should know that it is not unheard of to have the state police obtain a speed study, apply the 85th percentile rule and recommend a higher, not lower, speed limit.

Here is an interesting observation: RCOC traffic engineers report that based on past radar speed studies they have conducted in subdivisions, the 85th percentile ruling suggests that the speed limit on many subdivision streets should be 30 mph, not the 25 mph mandated by law. Apparently we don't even drive within the speed limit in our own neighborhoods. Let's face it, speed limits are only as good as the enforcement behind them. If you are concerned about speeding drivers, are you willing to pay more taxes for additional law enforcement?