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Speed adaption: An illusion we must deal with

October 22, 2010
Adirondack Daily Enterprise

Have you noticed how fast traffic moves when entering our Franklin County villages, where the posted speed limit is 30 mph?

Speed measurements show the speed averages in the 40s as vehicles enter our villages. There is a reason for this, and it is called "speed adaptation." Tom Vanderbilt, in his book "Traffic - Why We Drive the Way We Do," explains that the longer we drive at high speeds, the harder it is for us to slow down.

The reason is that neurons in the brain that track forward movement begin to become fatigued as a person looking ahead drives at the same speed for a time. The fatigued neurons begin to produce, in essence, a negative "output," which fools you into thinking you are moving slower than you actually are.

Because of this, when we go from 55 or 60 mph into a 30 mph zone, as we slow to the 40s, it feels like we are going much slower, perhaps the 30 mph speed limit. In essence, we underestimate our speed when asked to slow down and overestimate our speed when asked to speed up.

As Vanderbilt explains it, this is the reason why we often go too fast coming off a highway, it also explains why drivers entering a highway frequently fail to reach the speed of traffic by the time they are merging into traffic already on the highway, frustrating those in the right hand lane that are forced to slow down to let the driver merge.

So now that we have identified a real problem, how do we correct for it? We can use pilots for a good example. They rely on their instruments rather than on what it feels like, and for good reason. A pilot's instruments tell him whether the aircraft is turning, climbing, descending, air and ground speed, and much other information necessary for safe flying.

What we as drivers should do is similar - look at the speedometer. Don't rely on what speed it "feels like." You may be going much faster than what you think, and you may be going too slow for some conditions, such as a merge onto an expressway. It is also important to know the change in the posted speed zone you are leaving and the one you are entering, or perhaps the suggested exit ramp speed as posted on a warning sign as you exit an expressway.

Vanderbilt states that the reason we have speedometers, and why you should pay attention to yours, is that drivers often do not have a clue about how fast they're going - even when they think they do. A New Zealand study measured the speed of drivers as they passed children playing with a ball and waiting to cross the street. When questioned, drivers stated they were going at least 20 kilometers per hour slower than they really were. They thought they were doing 18 to 25 mph when they were really doing 31 to 37 mph. Point made!

The speedometer is accurate - our senses are not. This explains why, when changing from higher speeds to lower speeds, we can be fooled into thinking we have slowed more than we really have. Check the speedometer, so you'll know. Only a foolish, and perhaps fatal, pilot would rely on his senses over his instruments. Perhaps the same can also be said for a foolish driver.

For more articles on Vehicle and Traffic Law and traffic safety, visit the Traffic Safety Board's website at

Dave Werner can be reached at



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