Multiple passing maneuvers: new design and marking criteria to improve safety

Passing maneuvers can be single or multiple, depending on how many vehicles are involved. In a multiple pass, one passing (or fast) vehicle passes at least two impeding (or slow) vehicles, without returning to the own lane until the whole maneuver is completed. Consequently, the required gap in the opposing traffic is longer. Nevertheless, all passing sight distance criteria consider only single passing maneuvers, so they could be not enough to accommodate multiple passes. Besides, any author did not focused on the potential safety problem even though about 20% of observed passing maneuvers involved more than two impeding vehicles. An instrumented vehicle was used to collect data of single and multiple passing maneuvers in Spanish two-lane rural roads. The instrumented vehicle circulated at a constant speed, slightly lower than the operating speed, in order to be passed by faster vehicles. The whole passing maneuver, as well as the following process, was registered. External observations at certain locations complemented the observational data. The results provided guidelines to formulate a new passing sight distance model, valid also for multiple passes. Passing distance on the left lane was 43% higher for double maneuvers, and passing time on the left lane, 36% longer. On average, final speed of passing vehicle was 4 km/h higher in double passes. These results estimated the required passing sight distance. The comparison of the model with the existing criteria showed that passing zones with sight distance and length close to minimum values do not provide enough space to pass more than one vehicle with safety.