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.
Traffic Engineering 101
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.
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.
Yet most speed limits are set below the 85th percentile speed. We first investigated this topic at the urging of the National Motorists Association, a “member-supported driver advocacy organization” that has made raising speed limits to the 85th percentile one focus of its efforts.
One member pointed us to a 1992 report by the U.S. Department of Transportation on the “Effects of Raising and Lowering Speed Limits,” which, beside making the same arguments described above, noted that the majority of highway agencies set speed limits below the 85th percentile, leading over 50% of motorists to drive “in technical violation of the speed limit laws.” Lt. Megge believes the compliance rate in Michigan to be well under 50%.
It seems absurd that over half of drivers technically break the law at all times. It’s also perplexing that speed limit policy so consistently ignore traffic engineering 101. So why do people like Lt. Megge need to spend their time trying to raise speed limits?
by Alex Mayyasi, Pricenomics | Read more:
Image: NDF
[ed. Then there are places like Lynnwood, WA that love enforcing the posted speed limit. It's a great revenue generator.]
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.
Traffic Engineering 101
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.
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.
Yet most speed limits are set below the 85th percentile speed. We first investigated this topic at the urging of the National Motorists Association, a “member-supported driver advocacy organization” that has made raising speed limits to the 85th percentile one focus of its efforts.
One member pointed us to a 1992 report by the U.S. Department of Transportation on the “Effects of Raising and Lowering Speed Limits,” which, beside making the same arguments described above, noted that the majority of highway agencies set speed limits below the 85th percentile, leading over 50% of motorists to drive “in technical violation of the speed limit laws.” Lt. Megge believes the compliance rate in Michigan to be well under 50%.
It seems absurd that over half of drivers technically break the law at all times. It’s also perplexing that speed limit policy so consistently ignore traffic engineering 101. So why do people like Lt. Megge need to spend their time trying to raise speed limits?
by Alex Mayyasi, Pricenomics | Read more:
Image: NDF
[ed. Then there are places like Lynnwood, WA that love enforcing the posted speed limit. It's a great revenue generator.]
