Almost Every Speed Limit Is Too Low

Almost Every Speed Limit Is Too Low, by Quartz.

Over the past 12 years, lieutenant Megge [of the Michigan State Police] 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, lieutenant Megge advocates for raising speed limits because he believes it makes roads safer. …

Speed limits should and often do just reflect what people drive anyway:

Every year, traffic engineers review the speed limit on thousands of stretches of road and highway. … 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. …

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.” …

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. …

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 lieutenant Megge puts it, that “no one I know who gets into their car wants to crash.” …

So why have speed limits? To make the traffic flow faster. Yep, faster.

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 lieutenant Megge’s experience — and, more importantly, the variance in driving speeds.

This is important because, as noted in a US 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.” …

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.

But most speed limits are deliberately set too low:

Yet most speed limits are set below the 85th percentile speed. … 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.”

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 lieutenant Megge need to spend their time trying to raise speed limits? …

Consistently, the 85th percentile loses out to the perception that faster roads are less safe, so speed limits should be low. It’s a misconception, lieutenant Megge says, that he faces often in his work. When he proposes raising a speed limit, the initial reaction is always “Oh my god! You can’t do that. People are already going too fast.” People think raising the limit 10 mph will lead people to drive 10 mph faster, when really changing the limit has almost no impact on the speed of traffic. …

The other reason speed limits may remain low … is that cities and police departments use traffic citations as a revenue generating tool.