Observations by a Citizen: Speed Limits
By: Hal Rounds
Once upon a time, I worked with the National Motorists Association on speed limit litigation and legislative issues. One project I participated in as a volunteer was drafting a bill to bring Tennessee’s speed limit law (TCA 55-8-152 and 153) to conform to the federal highway regulations. (While I strenuously object to the myriad intrusions of federal government agencies into all sorts of activities that our United States Constitution forbids, the Constitution does authorize them “To establish Post Offices and post Roads.” (Article One, §8. “Establish” includes setting nationally uniform standards for speed limits.) So, when parts of U.S. Highway 64 in Somerville were reduced to 55 MPH, it got my attention.
What is the most effective way to set speed limits? Decades of research have repeatedly concluded that the best indicator of what speed is safe in any particular stretch of road is to watch what drivers tend to do on those stretches. Exceptions include where bad road engineering has kept hazards from being easily noticeable, or traffic flow inconveniently complex and inconvenient. But it is also necessary to have basic rules easy to remember and convenient to obey.
The federal “Manual on Uniform Traffic Control Devices”(“MUTCD”) addresses these factors. It sets two classes of speed limits: Statutory Speed Limits and Speed Zones.
The easy-to-remember Tennessee Statutory Speed Limit is set in TCA 55-8-152, for every road not otherwise posted, at 65 miles per hour. The exception for Interstates is the Statutory Limit of 70 MPH.
When the Statutory limit is inappropriate, “Speed Zones” maybe established by the local jurisdiction. But there is a limitation on this power: the MUTCD states that speed zones are to be “based on engineering studies.” And the engineering standard is that, “the speed limit that is posted within a speed zone should be within 5 mph of the 85th percentile speed of free-flowing motor vehicle traffic …”
Engineering studies have shown that the speed of most drivers tends to cluster at near the speed where 85 percent of drivers choose, for the combination of safety and sensible use of their time, travel. The higher speed drivers are a minority, and so are the slower ones. Studies also indicate that the slower ones are involved in more crashes. When speed limits are set significantly below that 85th percentile speed, there are more accidents. Was there an engineering study for our new speed zone?
Perhaps there is an incentive for lower speed limits that is stronger than – or at least added to – safety. Since drivers will tend to drive at a comfortable, safe, speed regardless of what is posted, reducing the posted speed limit generally leads to anincrease in ticket revenue for speeding.
I have already seen more police vehicles parked in the median in the new U.S 64 speed zone.
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