"...Changes in highway infrastructure that have occurred [in the U.S.] between 1984 and 1997 have not reduced traffic fatalities and injuries and have even had the effect of increasing total fatalities and injuries ... Other factors, primarily changes in the demographic age mix of the population, increased seat-belt usage, reduced per capita alcohol consumption, and improvements in medical technology are responsible for the downward trend in total fatal accidents." "...Changes in highway infrastructure that have occurred [in the U.S.] between 1984 and 1997 have not reduced traffic fatalities and injuries and have even had the effect of increasing total fatalities and injuries... Other factors, primarily changes in the demographic age mix of the population, increased seat-belt usage, reduced per capita alcohol consumption, and improvements in medical technology are responsible for the downward trend in total fatal accidents."

Further Reading:

Traffic Fatalities and Injuries: The Effect of Changes in Infrastructure and Other Trends