entremet
Member
When Lieutenant Gary Megge of the Michigan State Police attends a meeting, he sometimes asks, ”How many of you broke the speed limit on your way here?"
Hearing his question, you might assume that Lt. Megge is a particularly zealous police officer. The type of person who walks half a city block to avoid jaywalking on an empty street. The model citizen who defers almost obnoxiously to the letter of the law. But that is not the point of Lt. Megge's question at all.
”We all speed, yet months and months usually pass between us seeing a crash," Lt. Megge tells us when we call to discuss speed limits. ”That tells me that most of us are adequate, safe, reasonable drivers. Speeding and traffic safety have a small correlation."
Over the past 12 years, Lt. Megge has increased the speed limit on nearly 400 of Michigan's roadways. Each time, he or one of his officers hears from community groups who complain that people already drive too fast. But as Megge and his colleagues explain, their intent is not to reduce congestion, bow to the reality that everyone drives too fast, or even strike a balance between safety concerns and drivers' desire to arrive at their destinations faster. Quite the opposite, Lt. Megge advocates for raising speed limits because he believes it makes roads safer.
One reason is that a minority of drivers do follow the speed limit. ”I've found that about 10% of drivers truly identify the speed limit sign and drive at or near that limit," says Megge. Since these are the slowest share of drivers, they don't affect the 85th percentile speed. But they do impact the average speed -- by about 2 or 3 mph when a speed limit is changed, in Lt. Megge's experience -- and, more importantly, the variance in driving speeds.
This is important because, as noted in a U.S. Department of Transportation report, ”the potential for being involved in an accident is highest when traveling at speed much lower or much higher than the majority of motorists." If every car sets its cruise control at the same speed, the odds of a fender bender happening is low. But when some cars drive 55 mph and others drive 85 mph, the odds of cars colliding increases dramatically. This is why getting slow drivers to stick to the right lane is so important to roadway safety; we generally focus on joyriders' ability to cause accidents -- and rightly so -- but a car driving under the speed limit in the left (passing) lane of a highway is almost as dangerous.
Traffic engineers believe that the 85th percentile speed is the ideal speed limit because it leads to the least variability between driving speeds and therefore safer roads. When the speed limit is correctly set at the 85th percentile speed, the minority of drivers that do conscientiously follow speed limits are no longer driving much slower than the speed of traffic. The choice of the 85th percentile speed is a data-driven conclusion -- as noted Lt. Megge and speed limit resources like the Michigan State Police's guide -- that has been established by the consistent findings of years of traffic studies.
More at the link. Pretty fascinating stuff. I've always though US speed limits were way too low.
https://priceonomics.com/is-every-speed-limit-too-low/