Predicting intentions not to "drink and drive" using an extended version of the theory of planned behaviour.

The aim of this study was to determine to what extent the theory of planned behaviour (TPB) extended with moral norm and descriptive norm is able to predict intentions not to drink and drive. We also wanted to examine whether different processes guide intentions among young drivers (35 years and below) versus drivers aged above 35 years, and women's versus men's intentions. Questionnaires were sent to a sample of 4000 people in Norway aged between 18 and 70 years, and were completed by 1025 respondents. Analyses were conducted among those who had a driver's licence and who reported drinking at least 1-2 times per year, resulting in a sample of 879 individuals (46.6% were men, 30.3% were 35 years and younger, M=43.9 years). The results showed that the TPB variables explained 10% of the variance in intentions in the sample as a whole, and that the extension variables added 2% to the explained variance after controlling for the impact of the TPB components. Perceived behavioural control was the strongest predictor of intentions (β=0.24, p<0.001), followed by descriptive norm (β=-0.12, p<0.001), attitude (β=0.09, p<0.01) and moral norm (β=0.08, p<0.05). Several group differences were found. The extended TPB model explained 16% and 5% of men's and women's intentions, respectively, and 26% and 9% of the variance in intentions among young and older drivers, respectively. The practical implications of these results for the development of interventions to encourage drivers not to drink and drive will be outlined.