Drivers searched for known symbols on a simulation of a dimly lit (5 1x) instrument panel. The symbols, from 2 mm to 25 mm diameter, were in groups of nine. They were randomly selected from 24 used to identify controls and displays on European Ford vehicles. The probability of recognition was related to size for eight symbols. This gives a rational basis for the size used in vehicles. No reliable difference was found between the performance of black symbols on white backgrounds and white on black. Substantial differences exist between the effectiveness of different symbols. Recommendations for improvements were based on confusions between symbols.