Setting The Right Speed Limit

The other day I was driving on a highway, tooling along on a clear and bright spring day during a non-rush hour period, when I came upon a little traffic snarl.  Road construction?  Rubber-necking past an accident site?  Nope.  It was somebody driving too slow in the passing lane, causing other cars in that lane to pile up behind him as he inched past the car in the slow lane.  Then people started to duck around the line of cars to see if they could pass by on the right.  Suddenly you had another illustration of my long-held view that the real problem on the highways is not your average speeders — that is, the people who routinely drive above the speed limit, as opposed to the drag racers or road ragers — but rather drivers who are driving too slow in the wrong place at the wrong time.

Traffic engineering studies that substantiate this belief.   When traffic engineers review the speed limits on road, they invariably try to set the limit using the “85th percentile” method: that is, finding the number at which 85% of drivers drive at or below the posted speed limit and only 15% exceed it.  Why?  Because while most people drive at what they think is an appropriate speed given the prevailing conditions — a speed that often doesn’t conform to the posted limits — there is a core of people who actually faithfully obey the speed limits.  If the posted speed limits are set too low for the “I’ll drive at an appropriate speed” crowd, you’ve created a situation where there is a significant speed variance between that group and the speed driven by the faithful obeyers.

And that is the point of maximum roadway danger, according to the U.S. Department of Transportation, which has concluded that “the potential for being involved in an accident is highest when traveling at speed much lower or much higher than the majority of motorists.”   As the article linked above notes, the 85th percentile approach is the traffic engineers’ method of threading the needle between the approaches of the appropriate speeders and the faithful obeyers:  “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.”

But here’s the rub:  setting speed limits to actually match the 85th percentile test would mean raising the speed limits on most American roads, because right now about 50 percent of drivers routinely exceed posted speed limits.  In short, the appropriate speeders are telling the traffic engineers that the posted limits are too low, so to get to the 85 percent figures engineers would need to increase the speed limits.  That move would be applauded by the majority of drivers, but lower speed limits are reflexively supported by safety advocates — and by towns that pad their budgets by issuing speeding tickets on the stretch of roadway that passes through the township limits.  Everybody who lives nearby knows where those speed traps are. but the out-of-towners get tagged with expensive tickets, pay them by mail, and help the town to hit its revenue projections.

Traffic issues are one of the great imponderables in modern society, where the experts say all of their data and experience points in one direction — raising speed limits — but political considerations work in the opposite direction and keep the limits below what most drivers would prefer.  On most roadways, that magic 85th percentile number remains a pipe dream that probably won’t be realized until the human factor is eliminated and we’re all being moved around by self-driving cars.