A police officer measured a skid mark to be 150 feet long. The driver said he was going under the speed limit, which was 45mph. Was the driver correct? How long would the skid mark have to be if he was indeed going 45mph when the brakes were applied?
According to one on-line skid calculator, even down a 6% grade on wet asphalt pavement the vehicle would probably have been going at least 49 mph to leave skid marks that long. On level dry road, the speed probably exceeded 56 mph.
Another on-line calculator gives the formula as
speed = √(30*CD*length)
where CD is a constant that depends on road conditions (surface, grade, tire condition). For dry level well-traveled asphalt, CD may be taken to be 0.68.
Solving for skid length, we get
length = speed^2/(30*CD) = speed^2/20.4
For 45 mph, this amounts to
length = 45^2/20.4 ≈ 99.3
At the reported speed, the skid marks would be expected to be 99 feet or less on dry level pavement.
Another on-line calculator gives the formula as
speed = √(30*CD*length)
where CD is a constant that depends on road conditions (surface, grade, tire condition). For dry level well-traveled asphalt, CD may be taken to be 0.68.
Solving for skid length, we get
length = speed^2/(30*CD) = speed^2/20.4
For 45 mph, this amounts to
length = 45^2/20.4 ≈ 99.3
At the reported speed, the skid marks would be expected to be 99 feet or less on dry level pavement.