Summary: This driving simulator study aimed to investigate (1) effects of peer passengers on a variety of risky driving measures, and (2) moderating effects of inhibitory control on these peer passenger effects. Two age groups (n = 30, 17-18 year-olds; n = 20, 21-24 year-olds) participated. Each participant completed two 28km test-drives in a medium-fidelity driving simulator. In the first drive, participants were asked to drive as they normally do. In the second drive, participants again were asked to drive as they normally do, now in the presence of a peer passenger. Measures of risky driving were: standard deviation of lateral lane position (SDLP), collisions with road hazards, speeding, and red light running. The results showed: (1) that peer presence can have negative ('risk increasing') but also positive ('protective') effects on driving performance, depending on the specific driving measure: whereas red light running increased, the number of collisions and SDLP decreased with peer passengers; (2) a moderating effect of inhibitory control on the peer passenger effect of speeding as (a) in a sub-group with low inhibitory control an increase in speeding occurred with peers, while (b) in a sub-group with high inhibitory control there was no effect of peers on speeding. This suggests that those with higher inhibitory control are more successful in resisting peer pressure.