Beltway Drivers Set Their Limit At Speed of Fright
By Courtland Milloy
Monday, August 18, 2003; Page B01
Driving along the Capital Beltway the other day, I noticed that the Maryland State Police had set up more speed traps than usual. I slowed down, eased into the right lane and decided to see what it would be like to do the posted speed limit -- 55 mph along this particular stretch.
It was as if I'd come to a stop. Cars were passing in a blur. I had become so much of a traffic hazard that even the most timid drivers were pulling out from behind me. Easing back up to 65 mph, I no longer felt like a menace. But it wasn't until I was doing about 70 that I really got back into the traffic flow.
So what gives: Fines for speeding -- or safety?
Several years ago, I appeared in D.C. traffic court on a speeding charge and was told that there is never a reason for exceeding the speed limit. I was fined and assessed penalty points on my driver's license, although they would be erased when I completed a course in driver safety.
In traffic school, however, the instructor told us that it was safer to go with the traffic flow, regardless of speed, than to be the odd man out on the road.
Go figure.
On the Maryland State Highway Administration's Web site, www.sha.state.md.us, a question is posed: Is it always safe to drive at the speed limit? The agency's answer: In adverse conditions, "motorists must adjust their speed according to the existing vehicle and pedestrian traffic, road surface, lighting and weather conditions. Maintaining a safe speed at all times is a serious responsibility shared by all motorists."
So what if that adverse condition is that everyone is already driving 10 mph faster than the posted limit?
Establishing a "safe speed" is the task of traffic engineers, who generally rely on a formula known as the "85th percentile speed" for guidance.
"The 85th percentile speed is the speed at or below which 85 percent of the motorists drive on a given road when unaffected by slower traffic or poor weather," the highway administration says. "This speed indicates the speed that most motorists on that road consider safe and reasonable under ideal conditions."
Does this mean that the speed limit could increase to, say, 100 miles an hour if 85 percent of motorists become even more harried and rushed than they are now?
"My gut says, 'No,' " said Gerson Alexander, a human factors consultant who specializes in highway design and driver behavior. "That's not going to happen because of an inherent nervousness that people are going to have when they are free to pick any speed they want. If you drive on the Beltway early on a Sunday morning, you don't see everybody going 80 or 90 miles an hour. There is some kind of internal mechanism in most drivers that strikes a balance between getting there fast and getting there safely."
David Buck, a highway administration spokesman, told me that when engineering studies on parts of the Beltway found that 85 percent of motorist were already doing about 67 mph in a 55 mph zone, the speed limit was raised to 65.
"The result was that fatalities decreased, not increased, as some had feared," Buck said.
Moreover, the Web site says: "Research has shown that the posted speed limit has little effect on the speeds at which most motorists drive. . . . Speed limits significantly lower than the 85th percentile speed are ignored by many drivers and difficult to enforce."
That doesn't stop the police from trying, however.
On the Beltway, I saw signs warning "Speed enforced from above." That didn't make me slow down; rather, I looked up and around -- briefly, of course -- for the spy plane.
Another sign warned that "aggressive driver imaging" was being used, meaning unmarked state police vehicles were on the prowl for road rage.
Still, I didn't see anyone going as slowly as I had while doing 55 miles an hour.
Here's an idea: Focus those aggressive imaging vehicles and spy planes on residential neighborhoods, where speeding is a real problem. As for the highways, pass a law banning slow drivers from the left lane and let's see how fast the 85 percent of us really want to go.
E-mail: milloyc@washpost.com