This study investigated the assertiveness and substance abuse as predictors of aggressive driving behaviour among private car owners in Ibadan, Oyo State, Nigeria. The study adopted cross-sectional survey design for data collection. A purposive sampling technique was used to select one local government area while convenience sampling technique was used to select 242 (males: 163; females: 79) private car owners. Data were collected using three instruments: Rathus Assertiveness Scale, Substance and Choice Scale, and Aggressive Driving Behaviour Scale while standard multiple regression was used to analyze the data. The results showed that assertiveness and substance abuse jointly predicted aggressive driving behaviour among the study population [R2 = 0.302, F (25.498), p < 0.05]. Also, substance abuse (β = 0.478, p <0.05) independently predicted aggressive driving behaviour among private car owners in Ibadan. However, assertiveness did not independently predict aggressive driving behaviour among the study population (β = 0.343, p < 0.05). The study tends to suggest that assertiveness and substance abuse are robust predictors of aggressive driving behaviour. Therefore, it is recommended that detecting device should be readily available to monitor drivers on the steering to detect whether or not they have used drugs while driving.