The aim of this study was to develop operating speed models for different vehicle classes for rural
two-lane highways. Ten horizontal curves, connected with relatively long tangents, with various
geometric characteristics, were selected to perfom this study. A series of traffic surveys were
carried out using automatic traffic counters, to collect sufficient speed data for different vehicle
classes. Preliminary analysis confimed the necessity of the classification of the vehicles in order
to develop operating speed models. Further analysis looked at the differences between speeds on
inside and outside lanes of curves and significant differences were found between the speeds of the
two directions. Several operating speed models were developed for tangents and curves for
different vehicle classes. These models are very useful and can be used for design consistency
evaluations.