Prior to the introduction of legislation in the United Kingdom, observational road-side studies showed that approximately 2 per cent of drivers use a mobile phone while driving. We studied the change in the usage rate of hand-held mobile phones from 10 weeks before to 10 weeks after the legislation came into force in December 2003. Across three different sites during the evening rush-hour, the usage rate changed from 1.85 to 0.97 per cent, a reduction of almost half. This change is attributed to the legislation. The effect of the legislation on accident and injury rates is unknown.