Drivers' headlight use: a lighting factoid

The relationship between the horizontal illuminance on the road and the number of vehicles using headlights is quantified for five different sites in upper New York State. The horizontal illuminance at which 50% of vehicles are using their headlights is significantly different for clear and cloudy sky conditions and for the different sites. The resulting road surface luminances at which 50% of vehicles have their headlights on are, for all sites and sky conditions, much higher than similar measurements carried out in the Netherlands in 1958.