1. Determine the speed in mph after the vehicle has stopped sliding. This might be zero, but generally won't be zero. If the vehicle strikes something at the end of the skid, it is still moving and this residual velocity must be allowed for when using the chart.
2. Move out (horizontally ) on a speed line until it hits one of the curves, the lower is for a 0.7 drag factor, the upper is for 0.8. This is the starting point for finding the speed drop associated with a given length of skid marks with a given residual speed after the mark ends.
3. Move out the length of the longest single skid mark, then move up until you hit the curve again to find the original speed.
a. A vehicle leaves 40' of skid marks before coming to rest- the original speed is about 30 mph for both drag factors.
b. A vehicle leaves 60' of skid marks before striking another vehicle, the damage and post impact slide suggest that its speed was 30 mph when it struck the other vehicle.
1. Move out from the 30 mph line until you hit the curve at about 40'.
2. Add the length of the skid mark -60'- go out to the 100' mark and then up until you hit the curve again at about 46-48 mph. This is the original speed of the vehicle.
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