Why did motorcyclist traffic deaths sharply decrease in 2009?

Purpose: Following a decade of continued growth, motorcyclist traffic deaths decreased by 16% in 2009. The prolonged business recession, volatile gas prices, changes in motorcycle helmet use and weather conditions may have contributed to the sharp fatality decline. Time series analysis will investigate the factor(s) responsible for the trend reversal. Data and methods: The time series analysis examines U.S. monthly motor vehicle traffic death counts reported in FARS, 1/1994-12/2009 (n=192). Four additional monthly predictor series are considered: (1) the civilian unemployment rate, (2) real average retail gasoline prices, (3) percent helmet use in fatal crashes, and (4) average surface temperatures. The time series analysis employs unobserved components models, a form of state space time series analysis. The models are estimated for the first 168 months (1/94-12/07) and validated using the following 24 months (1/08-12/09). Results: The time series analysis estimates that a $1 increase in real gasoline prices is related to a 31.4 (t=10.35) increase in monthly motorcyclist deaths. Helmet use is estimated to reduce motorcyclist deaths by 45% (t= -3.18), and a 1 degree increase in average surface temperatures is associated with 4.6 (t=4.81) additional monthly motorcyclist deaths. The unemployment rate is not statistically significantly related to motorcyclist deaths. The final model predicts well the decrease in motorcyclist deaths in 2009. Conclusion: Among the identifiable factors, declining gasoline prices contributed most to the sharp decrease in motorcyclist deaths in 2009. Lower gasoline prices may reduce motorcycling in favor of safer vehicle travel.