Safety belt effectiveness in preventing fatalities to drivers is examined versus 13 vehicle, accident or environmental factors by applying the double pair comparison method to appropriate subsets of the Fatal Accident Reporting System (FARS) data. For each factor safety belt effectiveness (the percent of fatally injured unbelted drivers who would not have been killed if they had been wearing belts) is estimated, as is an associated standard error of the estimate. The graphically presented results provide no evidence that safety belt effectiveness is systematically influenced by most of the factors investigated, including car mass and model year. The largest effect found was that safety belt effectiveness for single car crashes was (62 ±5)% compared to (30 ± 8)% for two-car crashes.