A Cochrane review (abstract , review ) included 24 studies with a total of almost 300 000 subjects. Nineteen trials reported traffic offences: pooled relative risk (RR) = 0.96, 95% CI 0.94 to 0.98), trial heterogeneity was significant (p=<0.00001). Fifteen trials reported traffic crashes: pooled RR = 0.98 (95% CI 0.96 to 1.01). Four trials reported injury crashes: pooled RR = 1.12 (95% CI 0.88, 1.41), trial heterogeneity was significant (p=<0.00001).