Opinion Columns & Blogs

The eighty-fifth percentile sets the speed limit

The website Priceonomics recently had an article about traffic speeds and the eighty-fifth percentile, which was a concept I had not heard of. After making calls to several traffic engineers and traffic pattern researchers, not only is it real, but it makes sense. Those of us in Middle Georgia can even see it in practice.

The eighty-fifth percentile is where more and more traffic engineers encourage states to set the speed limit. The concept, at its root, is that eight-five percent of people are driving a common speed, which typically is higher than the set speed limit. Set the speed limit to what those eighty-five percent of people are driving and traffic will not only flow smoother, but when coupled with enforcement of the left lane for passing only, it will reduce traffic collisions.

“Wait,” you might say. “Won’t people then just drive faster than the new limit?” Lots of studies say no. People have an innate sense of safety and drive no faster than is reasonable and prudent most of the time. It is why people slow down in storms to a more or less common speed.

This phenomenon has been in play on Interstate-75 through Macon whether you have realized it or not. I thought it was just me, but it turns out it is really happening. Though the speed limit on I-75 through northern Bibb County has increased to 70 mph, the average speed has not gone much above it. The average speed before and after the increase was roughly 70 mph.

A traffic engineer I spoke with at the Department of Transportation tells me it was somewhat expected. Heading south from Interstate-475, the reduction to two lanes pulls traffic more closely together and people tend to slow down. Add on an increase in 18-wheelers and traffic slows further. Northbound, traffic speeds up as people pass the Riverside Drive exit, but only after they get past the on-ramp for incoming traffic. The vegetation growth there could block a speed trap and people accelerate once they have cleared it.

Before that point, the cluster of traffic slows because of traffic in the right lane getting off at Riverside Drive. Northbound traffic between Pierce Avenue and Tom Hill, Sr. has increased because of the three lanes. It has increased somewhat southbound in that area, but not as much as had been anticipated.

People tend to have a sense of safety. In fact, collisions in the nation have been on the decline. According to Gary Megge of the Michigan State Police, quoted by Priceonomics, “We all speed, yet months and months usually pass between us seeing a crash….That tells me that most of us are adequate, safe, reasonable drivers. Speeding and traffic safety have a small correlation.”

There are a number of reasons states do not adopt the eighty-fifth percentile rule. People are convinced higher speeds lead to more vehicular collisions, so there is public resistance. Fuel economy is another reason for not increasing the speed limit. The biggest reason, however, is revenue. Speeders generate tickets, which generate revenue.

The Georgia Department of Transportation has set adjustable speed limits on Interstate-285 around Atlanta, which is a step in the right direction. The next step the state should consider is increasing the speed limit on Interstate-16, perhaps likewise making it adjustable. Seventy miles an hour as a maximum speed limit makes sense in most of the state, but the desolate stretch of I-16 could be a proving ground for the eighty-fifth percentile.

Erick Erickson is a Fox News contributor and radio talk show host in Atlanta.

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