Bob CollinsThere’s not a lot of new in today’s Minnesota Department of Transportation report on congestion on metro-area roadways. It’s getting worse, as anyone who’s tried to get anywhere on metro-area roadways can attest.
But one of the reasons it’s getting worse — aside from there being more cars — is the habit of drivers who contribute to the shock wave effect.
You can find it during any rush hour. In my case, I find it usually on Minnesota Highway 52 out of St. Paul on the top of the bluff, and on Interstate 94 on the tight left corner on the east side.
Why does traffic slow to a crawl, then suddenly speed up when there’s nothing in the road to account for the sudden stall?
It’s the “shock wave”, according to MnDOT and it usually happens when cars get to around 45 mph.
A shock wave is a phenomenon where the majority of vehicles brake in a traffic stream. Situations that can create shock waves include:
• Changes in the characteristics of the roadway, such as a lane ending, a change in grade or curvature, narrowing of shoulders, or an entrance ramp where large traffic volumes enter the freeway.
• Large volumes of traffic at major intersections with high weaving volumes and entrance ramps causing the demand on the freeway to reach or exceed design capacity.
• Traffic incidents, such as crashes, stalled vehicles, animals or debris on the roadway, adverse weather conditions and special events.
Drivers’ habits can also contribute to shock waves. Drivers’ inattentiveness can result in minor speed variations in dense traffic or sudden braking in more general conditions.
In these situations, shock waves move upstream toward oncoming traffic at rates varying according to the density and speed of traffic.
As the rate of movement of the shock wave increases, the potential for rear end or sideswipe collisions increases. Multiple shock waves can spread from one instance of a slowdown in traffic flow and blend together with other extended periods of “stop-and-go” traffic upstream. This condition is referred to as a “breakdown” in traffic.
Usually breakdowns last the remainder of the peak period if traffic volumes are close to or above design capacity. These types of breakdowns are typical in bottleneck locations on the freeway system.
This is how it happens:
There is a cure for this sort of phantom traffic jam. William Beaty, an electrical engineer, figured it out in 1996 and started TrafficWaves.com.
As with most things in traffic, the solution will make other drivers around you mad — it’s to drive slower, the BBC documented.
Beaty’s methods (which, really, boil down to “be nice, and don’t be in a hurry”) are catching on with some drivers, though it’s impossible to say just how much of an effect they’re having on your daily commute, since things like GPS navigation, increased load, and adaptive cruise control must be factored in as well.
Beaty is hopeful that drivers-ed teachers will add his theories to their curriculum, but he knows his biggest obstacle is drivers themselves, and their obsessive need to be somewhere sooner.
It makes perfect sense, really, and you’ve probably seen truckers do it, leaving plenty of room to the car in front, at least until people in a hurry to stop up ahead fill in the gap and cause a shock wave.
Eventually, once automated cars catch on, the biggest problem causing traffic jams — humans — will be eliminated.
In the meantime, try Beaty’s approach this afternoon. Especially if you’re on Highway 52 or I-94 on the east side.
