In his book Traffic, Tom Vanderbilt discusses why drivers unknowingly drive faster than they think they are. The information was taken from two studies.
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.
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.
Training
The implications for training are obvious. If the driver of an SUV thinks they are going 40 MPH and they are actually moving at 46 MPH – life will get exciting quickly. Speed needs to be monitored with a radar gun
The solution seems to be simple – frequently glance at your speedometer. That is OK on a highway with nothing in front of the driver but open space, and the goal is not to get a speeding ticket. BUT in city environment or in a two or three vehicle scenario with mixed vehicles, looking at the speedometer can be problematic.
A later discussion will talk about this phenomenon while driving in a convoy scenario.