Bob,
I specifically mentioned open interstate, in which case most of the factors in lowering the limit do not apply. The 85th percentile heuristic is not based just on this study; a number of studies have observed that the fewest accidents occur when speed limits are are set at or near the 85th percentile. It's definitely a "rule of thumb", but one that traffic engineers have been using for decades.
And it's not
my logic, it's the logic of the NHTSA, DOT, FWHA, and the traffic engineers that do the research for them. Here's the abstract from yet another government report on speed limits:
This study recommends a method to establish maximum speed limits based on the 85th percentile of travel speeds. The conclusion is supported by an extensive literature search and analysis of traffic flow data collected by a unique Computer-Sensor System. Such data indicate that risk increases with deviation from mean speed. Such increase is minimal until approximately the 85th percentile, when the slope of the risk curve starts to rise sharply.
Taken from "A Study for the Selection of Maximum Speed Limits." October 1970. Four (4) volumes, prepared for the National Highway Traffic Safety Administration (NHTSA), U.S. Dept. of Transportation, by the Indiana University's Institute for Research in Public Safety. IRPS report number FH-11-7275; Volume I is also federally numbered as PB 197 373, and DOD HS-800 378.
The guideline to set speeds at the 85th percentile is also outlined in the Manual for Uniform Traffic Control Devices (MUTCD). This is the Federal Highway Administration document that defines the standards used by road managers nationwide to install and maintain traffic control devices on all streets and highways. From
Chapter 2B, Section 2B.13, "Speed Limit Sign (R2-1)":
Standard:
After an engineering study has been made in accordance with established traffic engineering practices, the Speed Limit (R2-1) sign (see Figure 2B-1) shall display the limit established by law, ordinance, regulation, or as adopted by the authorized agency. The speed limits shown shall be in multiples of 10 km/h or 5 mph.
Guidance:
At least once every 5 years, States and local agencies should reevaluate non-statutory speed limits on segments of their roadways that have undergone a significant change in roadway characteristics or surrounding land use since the last review.
No more than three speed limits should be displayed on any one Speed Limit sign or assembly.
When a speed limit is to be posted, it should be within 10 km/h or 5 mph of the 85th-percentile speed of free-flowing traffic. (emphasis mine)
Just so the terminology is clear, "Standards" are required by the MUTCD, whereas "Guidance" is just that. Now, the guideline, based on the recommendations of traffic engineers, states that the 85th percentile rule should be used in determining speed limits. The standard, though, allows politicians to set the limit as they damn well please. What
I want to see is the standard in 2B.13 modfied to incorporate the engineers' guidelines, so that corrupt politicians seeking to line city or state coffers with speed limit fines have
no choice but to implement the
safest speed limit on the roads. I want limits determined by scientific and statistical principles and traffic surveys done by engineers, not determined by "officials" seeking the most profitable, or the most politically expedient limits.
Who knows more about how traffic flows? Traffic engineers, or bureaucrats and politicians? I know who I trust more . . .
Originally posted by rjstaaf:
You are proposing that since people are going to speed anyway, lets just make it legal.
Um, no. I'm proposing that speed limits be set based on the recommendations of engineers who have determined that, statistically, the 85th percentile speed is where the fewest accidents occur.
Tort