This study tested the efficacy of implementation intentions in the context of drivers' speeding behavior. Participants (N = 300) completed self-report measures of goal intention and behavior, and they were randomly assigned to an experimental condition, which required them to specify an implementation intention, or a control condition. One month postbaseline, self-reported compliance with speed limits significantly increased for experimental participants but not for control participants. The effects of specifying an implementation intention on behavior increased with the strength of drivers' goal intentions. Finally, analysis of participants' implementation intentions revealed that specifying more behavioral strategies increased the frequency with which participants reported complying with the speed limit. Implications of the findings are discussed in relation to enhancing road safety interventions.