There have been numerous studies of traffic crashes that analyze the relationship of crashes with traffic conditions, the road geometry, and environment. The present paper aims to systematically investigate the possible differences in the effects of those variables during night, twilight or day. Previous studies show there is a relationship between traffic crash frequency, injury severity, and the time of day. Also, research shows that driving is different depending on the time of day. Recent work using a driving simulator has found that there is a significant driving speed differential between night and day and concludes that roadway geometric conditions that are safe during day may not be safe during nighttime driving. Much of the research that considers differences between night and day driving focuses on driver-specific characteristics such as drunk driving, young drivers, age and gender, sleepiness, but omits a systematic investigation of daytime vs. night differences in road environment variables. The main contribution of this paper is therefore an investigation of the effect of road environment conditions on crashes under different light conditions, using random parameter Poisson and negative binomial regressions which are estimated separately for each light condition (daytime, nighttime, twilight) and the whole 24-hour day.