THE FORMULA MAKES CERTAIN ASSUMPTIONS ABOUT THE APPLICABILITY OF CLASSICAL LAWS OF FRICTION. IN PRACTICE, FORENSIC SCIENTISTS BASE THE VALUE OF THE COEFFICIENT OF FRICTION ON APPROXIMATIONS OF ROAD CONDITIONS. SUCH AN APPROACH FAILS TO RECOGNIZE THE WIDE VARIATION AMONG MODERN TIRES AND OVERSIMPLIFIES THE FRICTIONAL BEHAVIOR OF RUBBER IN GENERAL AND TIRES IN PARTICULAR. DATA ON NINE DIFFERENT BRANDS OF TIRES REVEAL CONSIDERABLE VARIATION IN COEFFICIENTS OF FRICTION. THE TIRE WITH THE POOREST STOPPING PERFORMANCE GENERATES A GREATER COEFFICIENT OF FRICTION THAN THAT USUALLY CITED. THE DATA ALSO SHOW CONSIDERABLE AND UNPREDICTABLE VARIATION BETWEEN FORWARD AND LATERAL TRACTION. ANOTHER MAJOR ASSUMPTION IN THE TRADITIONAL FORMULA IS THAT VEHICLE TYPE DOES NOT AFFECT THE OUTCOME OF VELOCITY DETERMINATIONS MATERIALLY. DATA ON APPARENT COEFFICIENT OF FRICTION BASED ON STOPPING DISTANCES FOR FOUR AUTOMOBILE MAKES SUGGEST THAT SUCH AN ASSUMPTION IS NOT WARRANTED. ANOTHER FACTOR RARELY CONSIDERED IN APPLYING THE FORMULA IS LOCAL VARIATION IN PAVEMENT SURFACE (PATCHES, PAINT, MANHOLE COVERS, ETC.). IT IS CONCLUDED THAT, IN LIGHT OF AUTOMOBILE AND TIRE TESTS AND EVIDENCE PERTAINING TO THE DIVERGENT FRICTIONAL PROPERTIES OF RUBBER, THE TRADITIONAL VELOCITY FORMULA APPEARS TO BE INADEQUATE. A BIBLIOGRAPHY IS INCLUDED. (LKM)