Exploring the age discrepancy in death rates from motorcycle injury in the United States of America: the decomposition method

Introduction Motorcyclist deaths accounted for 13% of all traffi c deaths in 2009 in the United States of America. Previous research has suggested age discrepancy among motorcycle drivers. We examined the various factors that contribute to this age discrepancy.

Methods The decomposition methodology separates the individual components contributing to overall rates. It has been used to explore differences in injury death rates across population groups, time periods, and geographic regions. The populationbased motorcycle driver fatality rate (A: # motorcycle driver fatalities / # personyyears) can be expressed as the product of the driving exposure (B: # miles driven / # personyears), crash risk (C: # crashes / # miles driven) and crash fatality rate (D: # motorcycle driver fatalities / # crashes). Using ages 4049 as the referent group. We expressed the comparison of fatal crash involvement rates between the referent group and those aged 2029 years as a ratio: A2029 /A4049 = B2029 / B4049 × C2029 / C4049 × D2029 /D4049 The relative contribution (RC) of each component (B, C, or D) to the difference in motorcycle driver fatality rate is: RCi = |In(RCi)| / {|In(RCb) + |In(RCc)| + |In(RCd)|} × 100%, where i = b, c or d; In: natural logarithm Data sources included the 20082009 Fatality Analysis Reporting System (FARS), General Estimates System (GES), National Household Travel Survey (NHTS), and resident population estimates. FARS is a cencus of fatal crashes in the United States. GES is a nationally representative sample of policereported crashes. Respondents in the NHTS were instructed to keep a written diary of all the trips made during a randomly assigned 24hour travel day: information included trip prupose, transportation means, and trip length. The estimates of miles driven were obtained on the day trip diary.

Results Relative to ages 4049 years, the ratio of populationbased fatality rate was 0.39, 1.14, 0.93, 0.95, and 0.40 for ages of 1619, 2029, 3039, 5059, and 60 and over. Compared with persons aged 4049 years, the ratio of average annual miles driven was 0.13, 0.32, 0.41, 0.80, and 0.27 for ages 1619, 2029, 3039, 5059, and 60 and over; the ratio of crash risk was 4.81, 3.72, 2.40, 1.22 and 1.08 for ages 1619, 2029, 3039, 5059, and 60 and over; the ratio of crash fatality rate was 0.64, 0.96, 0.96, 0.97, 1.37 for ages 1619, 2029, 3039, 5059, and 60 and over. The relative contribution of driving exposure and crash risk was equally important for ages 2029, 3039, and 5059, while the relative contribution of crash fatality rate was 19% for age 60 and over.

Conclusions Motorcyle drivers aged 4049 years had the highest average annual miles driven and the lowest crash risk. For 2029 and 3039 year olds, reduced driving offset their high crash risk, making their fatality rate comparable to that for ages 4059. Prevention practice should focus on safety training and crash avoidance for motorcycle drivers aged 1619, 2029, and 3039 years to reduce their elevated crash risk.