You’re heading down I-77 or I-85 when suddenly you find yourself in bumper-to-bumper traffic. For the next 20 minutes you creep along at a maddening stop-and-go pace, craning your neck to get a look at the accident or construction that must surely be causing the congestion. Then, as suddenly as it started, the traffic jam clears, and you’re on your way again without ever passing so much as a fender bender or one orange cone.
So what gives? What causes these traffic jams that appear just as mysteriously as they disappear. For an answer, take a look in the rearview mirror — that’s right bucko, it’s your fault.
According to traffic-flow specialists, these mysterious traffic jams are caused by something known as the shock-wave effect. Highway drivers, according to the experts, operate at peak efficiency around 35 mph, and are capable of performing satisfactorily at higher speeds. What typically happens, though, is that when traffic volume on a highway nears its designated optimum capacity, you’ll always have a couple of boneheads who drive under 35 mph for whatever reason (they’re tired, their medication kicked in, they saw a UFO, etc.). These slower drivers set off the dreaded shock-wave effect. First, otherwise speedy and competent drivers (i.e., you and me) react irrationally when the pace slows on the highway. They (or we) start looking for the reason they had to slow down in the first place, and they begin to overreact to any stim
uli, particularly the brake lights of the cars ahead of them. Soon you’ve got a number of drivers behaving in this hypersensitive manner, setting off a shock wave for miles behind them, and creating bumper-to-bumper traffic for no apparent reason — not to mention a litany of swear words.
So how do these shock-wave traffic jams suddenly disappear? Usually traffic clears because there is a smaller ratio of traffic volume to capacity ahead. In other words, there’s usually enough breathing room to prompt even slowpoke victims of the shock-wave effect (i.e., grandma and grandpa) to peel away at 35 mph or more. *
This article appears in May 8-14, 2002.
