#### Determining Vehicle Speeds From Skid Marks

30 = A constant value used in this equation. D = Skid Distance, in decimal feet and inches. f = Drag factor for the road surface. n = Braking efficiency as a percent. An example: A car skids to a stop, leaving four skid marks with an average length of 60 feet. The road is asphalt. Skid tests reveal a drag factor of