The Highway Capacity Manual (HCM 2010) is used in Spain to analyze traffic operation and quality of
service. The effect of passing in two-lane highways is considered through adjustment factors to the
average speed and the percent time spent following. They depend on the percentage of no passing
zones and were calibrated with traffic simulations based on field data from the 70’s.
The objective of this research is to evaluate the performance measures in two-lane highways
considering the distribution of passing zones. TWOPAS was calibrated and validated with field data. For
the calibration, genetic algorithms were used. Then, the model was applied to an ideal two-lane highway
varying the traffic flow and the passing zone distribution. More than 85.000 scenarios were simulated.
The percentage of no passing zones was not enough to explain the effect of passing maneuvers in the
performance measures; so, the length of the passing zones should also be considered
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