The efficacy of an extended theory of planned behaviour (TPB) was tested in relation to offending drivers' (N=1403) speeding behaviour. Postal questionnaires were issued at Time 1 to measure intention, instrumental and affective attitude, subjective and descriptive norm, self-efficacy, perceived controllability, moral norm, anticipated regret, self-identity, and past speeding behaviour. At Time 2 (6 months later), subsequent speeding behaviour was measured, again using self-completion postal questionnaires. The extended TPB accounted for 68% of the variation in intention and 51% of the variation in subsequent behaviour. The independent predictors of intention were instrumental attitude, affective attitude, self-efficacy, moral norm, anticipated regret and past behaviour. The independent predictors of behaviour were intention, self-efficacy, anticipated regret and past behaviour. Theoretical and practical implications of the findings are discussed in relation to targeting road safety interventions.