Association between older drivers' signs and motor vehicle crashes in Japan.

Background: Encouraging older drivers to display the older drivers' sign is one of the unique traffic safety policies for older drivers in Japan. The sign was discussed in light of ageism, but the potential merit of displaying it has not been investigated.

Methods: Using nationwide police-reported traffic crash data, we conducted a case-control study to examine whether displaying the sign is associated with reduced rear-end collision risk among drivers aged 70 years or older who were involved in car-to-car collisions from 2014 to 2023. The cases were those involved in rear-end collisions, the controls were those involved in angle collisions, and the exposure of interest was the sign display, which should be less often observed in the cases if the sign display is effective. The association was assessed with odds ratios adjusted for the time of crash and drivers' sex and age group.

Results: Among 74,433 cases and 13,885 controls, the proportion of those displaying older drivers' signs was 38% and 39%, respectively. By the time of crash and drivers' sex and age group, the proportion tended to be slightly higher in the cases than in the controls. In both cases and controls, the proportion was higher among women, in older age groups, and during daytime. The crude and adjusted odds ratios were 0.99 (95% confidence interval [CI]: 0.95-1.02) and 1.08 (95% CI: 1.04-1.12).

Conclusion: Displaying the older drivers' sign was not associated with reduced rear-end collision risk. Alternative interventions are needed to ensure their traffic safety.