Effects of single oral doses of codeine, diazepam (Valium), and alcohol on simulated driving were investigated by using a modification of the English Sim‐L‐car. The driving time was 40 minutes, subjects were told to adapt speed to surroundings and traffic. Placebo increased the inaccuracy of speed estimations. Alcohol increased the numbers of steering wheel reversals and neglected instructions. Diazepam 10 mg increased the number of collisions and neglected instructions, but the greatest increase in collisions was after codeine 50 mg. Diazepam generally enchanced the effect of alcohol.