The ability of psychosocial variables to predict driver behaviour was tested using the theory of planned behaviour (TPB; I. Ajzen, 1985) as a theoretical framework. At Time 1, participants (N=150) completed questionnaire measures of TPB variables. 1 week later, participants' behaviour was assessed using both self-reports and observations of driving speed derived from a high-fidelity driving simulator. Multiple regression analyses demonstrated that: (a) the TPB was a strong predictor of drivers' intentions and self-reported speeding behaviour, and (b) intention was the sole predictor of observed speeding behaviour. Standard and repeated events survival analyses showed that intention also predicted the maintenance of drivers' compliance with speed limits. The discussion focuses on the TPB's relationships with observed and self-reported behaviour, and the implications for designing interventions.