In his book Traffic, Tom Vanderbilt discusses why drivers unknowingly drive faster than they think they are. The information was taken from two studies.
The Driver’s Eye Height
A study was conducted to assess drivers’ chosen speed when they operated a simulated vehicle while viewing the road from a low eye height and a high eye height. Participants were instructed to drive, without reference to a speedometer, at a highway driving speed at which they felt comfortable and safe. Drivers seated at a high eye height drove faster than when they were seated at a low eye height.
The study mentioned that SUV drivers are often reported to be overconfident, tending to overestimate their vehicles capabilities. Evidence suggests that because they sit higher, drivers of SUVs (and vans and pickups) are less able to judge speed accurately.
A Drivers Perception of Speed
A study on the perception of speed found that the “average” driver is 15% off; they are driving 15% faster than they think they are. So if you think you are driving at 40 MPH you are more than likely driving 46 MPH. If you think you are driving at 60 MPH you are probably going 69 MPH. And the higher you sit in the vehicle the worse it is.
To carry this one step further, if the driver is moving 15% faster than they think they are that 15% would represent an increase of 30% in the energy the driver will need to manage in an emergency maneuver, and an increase in 30% in the distance required to stop. As the vehicle approaches the limit a difference of 2% to 5% can make the difference between control and no control.
Former students know that we are sensitive to this phenomenon. In our first driving exercises – the slalom – students most often are driving faster than they estimate. In the SUV program it is amplified. We use a radar gun to monitor their speed, and relay the information back to the driver. In the process the driver is able to compare speed vs. output, and within a small amount of time is able to accurately determine speed without looking at the speedometer.
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