Examining Correlation and Trends in Seatbelt Use among Occupants of the Same Vehicle using a Bivariate Probit Model

Research has consistently demonstrated that seatbelt use is critically important in reducing the likelihood of fatal and serious injuries resulting from traffic crashes. However, after years of nationwide increases in seatbelt use, these rates have largely plateaued, motivating the need for research to better understand those circumstances under which seatbelt use remains relatively low. At an aggregate level, research has shown that occupants in the same vehicle tend to exhibit correlation in seatbelt use or non-use. This suggests that social dynamics may play a role in occupants’ decisions as to whether or not to wear a seatbelt. To that end, this study examines trends in seatbelt use among pairs of drivers and front-seat passengers using data from direct observation roadside surveys. Bivariate probit models are estimated to examine the relationship between seatbelt use and various demographic, vehicle, and site-specific factors. The bivariate framework is also able to account for correlation among important unobserved factors associated with seatbelt use. The results show significantly better fit as compared with independent univariate probit models. The results also suggest both direct and indirect relationships between seatbelt use and various demographic, vehicle, and site characteristics. Seatbelt use rates are found to vary based on occupants’ age, gender, and race. Furthermore, seatbelt use by both the driver and front-seat passenger is also shown to vary based on the other occupant’s age. Heterogeneity is also shown across various geographic regions and roadway functional classes.