Motor vehicle deaths in the U.S. declined from nearly 60,000 in 1966 to just over 40,000 in 2011, even though Americans drive millions more miles now and millions more vehicles are on the road. The death rate, which was 7.6 deaths per 100 million miles in 1950, declined from 5.5 in 1966 to 1.6 in 2011. This _____________ can be attributed to the manufacture of safer vehicles with features such as airbags and antilock brakes.