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About the most firmly established vehicle-safety effect is that the heavier the vehicle, the lower are the risks to its occupants. Empirically data show that the additional mass of a passenger reduces driver fatality risk by 7%. While occupants of heavier vehicles enjoy increased safety, there are two important negatives associated with heavier vehicles. First, they increase risk to occupants of other vehicles into which they crash. Second, they consume more fuel. The size, or length, of a vehicle also affects safety. All other factors, including mass, being equal, a larger vehicle reduces fatality risk to its occupants. But unlike mass, it also reduces risk to occupants in vehicles into which it crashes. A quantitative relationship expressing fatality risk as a function of the mass and size of both cars involved in a two-car crash was derived in Causal influence of car mass and size on driver fatality risk, Am J Pub Health. 91:1076-81;2001.