The part played in traffic safety by illness or disability is unknown, as is the identity and degree of the disorders which necessitate the use of driving aids or completely incapacitate a person from driving. By means of a mock car, 28 persons suffering from Parkinson's disease were compared with 109 healthy controls. Only patients in presumed optimal drug regimen and without complicating disorders were included in the study group. The main results were failure to react to stimuli on several occasions, a high frequency of erroneous reactions in particular directional errors, reduced strength and speed of movement and increased reaction times. Typically the latter would entail a prolongation of the reaction distance with more than 1/3, i.e. 6 m when driving a car at a speed of 80 km/h. The Webster score proved to be an unreliable predictor of the results of the mock car test, and the UPDRS is suggested for future studies.