Effectiveness of primary and secondary enforced seat belt laws

Objective: The objective of this study was to determine the relative effica cy of primary and secondary enforced motor vehicle occupant restraint laws on the outcomes of restraint use, crash-related mortality, and crash-relate d injuries. Search strategy: We used the Cochrane Collaboration search strategy to sear ch the following electronic databases: MEDLINE, EMBASE, National Technical Information Service (NTIS), Psyc-INFO, ERIC, Nursing and Allied Health (CIN AHL), Transportation Research Information Service (TRIS), and EI Compendex. The reference lists from each potentially eligible study were checked, and knowledgeable people in the field were contacted, for additional leads to published reports. Selection criteria: Studies had to include a comparison of primary enforcem ent law to no law, secondary enforcement law to no law, or a primary to a s econdary law. Any study design was acceptable. Acceptable outcome measures included observed restraint use, and counts or rates of deaths or serious i njuries. Data collection: Data were collected using a standard abstract reporting fo rm. Relative differences in outcomes and absolute differences were calculat ed when possible. Main results: We identified 48 studies for the review. When places or time periods with primary enforcement laws were compared to those without such l aws, the relative prevalence of seat belt use ranged from 1.5 to 4.5; the p revalence differences ranged from 10 to 50 per 100 observed drivers. Second ary laws had smaller effects. Two studies evaluated a change in law from se condary to primary enforcement; this was associated with an increase in bel t use 6 months later of 5.3 per 100 observed drivers in Louisiana and 18 pe r 100 drivers in California. Primary enforcement laws were associated with a relative risk of death in MV crashes of .54 to .97. The reduction in mort ality associated with secondary enforcement laws was much more modest, with relative risks estimates of .81 to 1.025. Primary enforcement laws were as sociated with a relative risk of severe injuries of .20 to .89; the associa tion of secondary enforcement laws with severe injuries was smaller. Conclusions: Our review of existing studies suggests that primary enforceme nt laws are likely to be more effective than secondary laws. However, few s tudies are of good quality, and quantitative estimates of the relative effe ct of primary compared with secondary laws are limited.