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Background: Few studies have looked at how obesity affects injury outcomes among vehicle occupants involved in traffic collisions., Objective: To estimate the association of obesity with death risk among drivers of passenger vehicles aged ≥16 and to examine effect modification by driver sex, driver seat belt use, vehicle type and collision type., Methods: We conducted a matched-pair cohort study using data from the Fatality Analysis Reporting System. WHO body mass index (BMI) categories were calculated. Data were analysed using conditional Poisson regression., Results: Estimated risk ratios (RRs) were slightly raised for underweight drivers (RR=1.19, 95% CI 0.86 to 1.63). RR increased with higher BMI categories and were 1.21 (0.98 to 1.49) for BMI 30-34.9, 1.51 (1.10 to 2.08) for BMI 35-39.9 and 1.80 (1.15 to 2.84) for BMI ≥40. Estimated BMI effects varied by gender. We found no meaningful variation across levels of vehicle type, collision type or seat belt use., Conclusions: Findings from this study suggest that obese vehicle drivers are more likely to die from traffic collision-related injuries than non-obese occupants involved in the same collision. Education is needed to improve seat belt use among obese people, as is research to understand the potential role of comorbidities in injury outcomes.

Objective: To determine whether either the inclusion of adults in matched cohort studies of passenger vehicle occupants or modification of age effects by collision severity biases child restraint risk ratios biases estimate of child restraint effectiveness., Methods: Monte Carlo data simulations were conducted to represent 10,000 collision-involved vehicles carrying a mix of children and adults. The effects of age category, adult seat belt use, child seat belt use, and child safety seat use were set to known values. Age was a modifier of the adult and child seat belt risk ratios and of the safety seat RR, and crash severity was a modifier of the age RRs. We also created an association between restraint use and collision severity to confound the restraint-death risk associations. RRs were estimated using conditional Poisson regression and compared with the true values., Results: Estimated RRs for death were identical to the simulated values. The average child safety seat RR was 0.42 (true value 0.42), and the average seat belt RRs for children were 0.54 (true value 0.54) for those aged 0-3 years and 0.61 (true value 0.61) for those aged 4-17. Age effects were also correctly estimated, 1.69 (true value 1.69) for those aged 0-3 and 1.35 (true value 1.35) for those aged 4-17., Conclusions: Neither the use of matched sets of adults and children nor the modification of age effects by collision severity biases estimates of child restraint effectiveness in matched cohort studies.

Objective: To estimate the effectiveness of booster seats and of seatbelts in reducing the risk of child death during traffic collisions and to examine possible effect modification by various collision and vehicle characteristics., Methods: A matched cohort study was conducted using data from the Fatality Analysis Reporting System. Death risk ratios were estimated with conditional Poisson regression, bootstrapped coefficient standard errors, and multiply imputed missing values using chained equations., Results: Estimated death risk ratios for booster seats used with seatbelts were 0.33 (95% CI 0.28 to 0.40) for children age 4-5 years and 0.45 (0.31 to 0.63) for children aged 6-8 years (Wald test of homogeneity p<0.005). The estimated risk ratios for seatbelt used alone were similar for the two age groups, 0.37 (0.32 to 0.43) and 0.39 (0.34 to 0.44) for ages 4-5 and 6-8, respectively (Wald p = 0.61). Estimated booster seat effectiveness was significantly greater for inbound seating positions (Wald p = 0.05) and during rollovers collisions (Wald p = 0.01). Significant variability in risk ratio estimates was not observed across levels of calendar year, vehicle model year, vehicle type, or land use., Conclusions: Seatbelts, used with or without booster seats, are highly effective in preventing death among motor vehicle occupants aged 4-8 years. Booster seats do not appear to improve the performance of seatbelts with respect to preventing death (risk ratio 0.92, 95% CI 0.79 to 1.08, comparing seatbelts with boosters to seatbelts alone), but because several studies have found that booster seats reduce non-fatal injury severity, clinicians and injury prevention specialists should continue to recommend the use of boosters to parents of young children.

Objective: This study examines the association of nighttime driving and the carrying of passengers with the rate of motor vehicle crashes that resulted in severe or fatal injury to young drivers in California before the implementation of a graduated licensing system., Method: Passenger vehicle drivers aged 16 or 17 involved in injury crashes in California from 1 January 1993 to 30 June 1998 were identified through a police crash database. An induced exposure method was used to estimate driving exposure. Odds ratios for driver injury crashes were estimated with logistic regression., Results: Driving at night, driving without adult supervision, driving with passengers, using alcohol, being 16, and being male were associated with high rates of driver injury crash., Conclusions: The injury crash rate for drivers aged 16 or 17 increases during nighttime hours and in the absence of adult supervision, with or without other passengers. Driving between 10 pm and midnight is particularly dangerous for young drivers. Nighttime driving restrictions that begin at 10 pm or earlier and restrictions on carrying passengers at any hour may increase the effectiveness of graduated licensing systems.