Random Effect Models to Predict Operating Speed Distribution on Rural Two-Lane Highways

This paper presents the results obtained from the estimation of free-flow speed on two-lane rural highways. The data used for the analysis were collected in Northwest Italy using video cameras and a laser speed gun. The model structure adopted separates the estimate of the central tendency of speeds from the typical deviations of individual speeds. Hence, in the model, the same set of variables can be used to determine both the mean value and the standard deviation of the speed distribution; the desired speed percentile is then calculated by considering the associated standard normal random variable (Z). Random effects (RE) were included in the model to account for the variability in time and space of the data that contain multiple measurements for the same road/section/direction and to remove any dependency between estimation errors from individual observations.