The faster a car is traveling, the less time the driver has to avoid a potential accident, and if a car does crash, higher speeds increase the risk of a fatality. Between 1995 and 2000, average highway speeds increased significantly in the United States, yet, over that time, there was a drop in the number of car-crash fatalities per highway mile driven by cars.
The statement 'There were increases in both the proportion of people who wore seat belts and the proportion of cars that were equipped with airbags as safety devices' most helps to explain why the fatality rate decreased in spite of the increase in average highway speeds in the United States between 1995 and 2000.