The need for changes in the criteria for passing sight distance (PSD) used for design of two-lane highways and for marking of passing and no-passing zones on two-lane highways was investigated. PSD models from the AASHTO Green Book and the Manual on Uniform Traffic Control Devices were first compared with models developed in previous research to determine the most accurate representation of the passing maneuver. Two existing models warranted further consideration. Field studies were then conducted in Missouri and Pennsylvania; recent field data from a study in Texas were also evaluated to validate the assumptions made in the passing models. On the basis of current driver behaviors, no change was needed in the PSD criteria used for marking passing and no-passing zones. The research found that two-lane highways could be safely designed with PSD criteria equal to or greater than the PSD criteria used in marking passing and no-passing zones, instead of the longer PSD values currently used in design. Therefore, the use of the current PSD marking criteria in design is recommended. This practice will provide desirable consistency between PSD design and marking practices.