A car skids 60 feet. Several things must be determined before you can determine the vehicle's skid speed. You must know the skid distance, a drag factor for the road surface and the braking efficiency of the vehicle. This will be a conservative value as the wheels do not lock-up instantly. Shadow skid and clearly visible skid should be considered as one continuous mark for any given tire.
Cars have four tires, two in the front followed directly by the two in the rear.
The wheels on most cars, assuming the brake system is functioning correctly, will tend to lock at nearly the same time. Current brake design includes pressure limiters that prevent the rear wheels from locking before the front wheels lock. If all four wheels lock at the same time, and the vehicle is skidding in a straight line, the marks from the rear wheels will overlap the marks from the front wheels. Rear wheel skid marks can be identified by the dark center while skid marks from the front wheels can be identified by two distinct thin lines on the outer edges. If four distinct skid marks can be found, they should be measured individually.