Raising Speed Limits to Make Roads Safer

Here’s a piece on raising speed limits to make roads safer that will sound counterintuitive, at least to those who have never actually driven on an freeway:

“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.

Every year, traffic engineers review the speed limit on thousands of stretches of road and highway. Most are reviewed by a member of the state’s Department of Transportation, often along with a member of the state police, as is the case in Michigan. In each case, the “survey team” has a clear approach: they want to set the speed limit so that 15% of drivers exceed it and 85% of drivers drive at or below the speed limit.

This “nationally recognized method” of setting the speed limit as the 85th percentile speed is essentially traffic engineering 101. It’s also a bit perplexing to those unfamiliar with the concept. Shouldn’t everyone drive at or below the speed limit? And if a driver’s speed is dictated by the speed limit, how can you decide whether or not to change that limit based on the speed of traffic?

The answer lies in realizing that the speed limit really is just a number on a sign, and it has very little influence on how fast people drive. “Over the years, I’ve done many follow up studies after we raise or lower a speed limit,” Megge tells us. “Almost every time, the 85th percentile speed doesn’t change, or if it does, it’s by about 2 or 3 mph.”

As most honest drivers would probably concede, this means that if the speed limit on a highway decreases from 65 mph to 55 mph, most drivers will not drive 10 mph slower. But for the majority of drivers, the opposite is also true. If a survey team increases the speed limit by 10 mph, the speed of traffic will not shoot up 10 mph. It will stay around the same. Years of observing traffic has shown engineers that as long as a cop car is not in sight, most people simply drive at whatever speed they like.

Luckily, there is some logic to the speed people choose other than the need for speed. The speed drivers choose is not based on laws or street signs, but the weather, number of intersections, presence of pedestrians and curves, and all the other information that factors into the principle, as Lt. Megge puts it, that “no one I know who gets into their car wants to crash.”

So if drivers disregard speed limits, why bother trying to set the “right” speed limit at all?

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.

Speed traps and the Arab oil embargo also get mentioned.

(Hat tip: The Corner.)

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2 Responses to “Raising Speed Limits to Make Roads Safer”

  1. roger says:

    Back in the last century in the days of the double nickle I read of a study on “natural speed limits” by blinding speedometers on several different vehicles. IIRC, the average speed was around 75 mph. I don’t remember the variance except for one observation: the higher the driver’s eye the faster he drove (a van was the fastest-a corvette the slowest) as if the drivers were attempting to maintain about the same visual angular velocity of the road passing by. Again IIRC, the study was done by Texas A&M.

  2. MPH says:

    I used to commute in the Detroit area (Mound Blvd in particular). There was a one mile stretch that had a 35 MPH speed limit (due to a school) while the rest was 50. Eventually they set that mile to 50, but put in a school zone speed of 25 “when flashing”, so you only had to slow down when the kids were arriving and leaving. Because of when I was typically commuting, I usually didn’t need to slow down. Over 1 year after the limit had been raised, there were still idiots who would slow down to 35 for that mile. Since the lights were timed for 50, this caused problems with traffic flow. Sorry people, but if you haven’t noticed a change in traffic signs after a year, you’re too unobservant to be allowed to drive.

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