Most national highways in rural parts of Hokkaido, Japan, are two-way, two-lane roads. On these highways, passing maneuvers that involve the use of the oncoming lane are frequently observed. Such passing is observed even when road surfaces are covered with compacted snow. To optimize speed and safety, simulated passing maneuvers have been conducted to make use of the oncoming lane of a two-way, two-lane highway. No such traffic simulations, however, have taken into consideration the surface conditions of roads in winter (e.g., snow-covered, icy, with poor visibility caused by snowfall and fog). In this study, a traffic flow simulation to model passing maneuvers that involved the oncoming lane was created through the use of values measured in a passing-maneuver field survey conducted on a two-lane road in summer and winter. The field values for the number of passing maneuvers, traffic volume, and speed distribution were compared with the simulation results. The outcomes confirmed that the difference between them was small. The sensitivity analysis results revealed that the number of passing maneuvers on surfaces covered with compacted snow under conditions of poor visibility did not increase as much as they did on dry surfaces, even when the traffic volume in the original lane was higher. The passing success rate tended to decrease with higher volumes of traffic in the oncoming lane, regardless of surface conditions.

Hokkaido, Japan, is a cold, snowy island where winter lasts for approximately 5 months, from November to March. Road surfaces are usually dry, but frequently covered with compacted snow. A measure to improve existing two-lane highways to two-plus-one (2+1) lane highways by installing an auxiliary lane has been introduced to offer a better quality of service to road users. A sensitivity analysis was undertaken with a traffic flow microsimulation program, SIM-R, to evaluate the effectiveness of 2+1-lane highway sections, which were built by adding an auxiliary lane to rural two-lane highways, in a cold, snowy region. The road surface conditions under analysis were dry and covered with compacted snow. The hourly traffic volumes were changed from 100 to 1,000 vehicles per hour. Evaluation indicators were the average travel speed, the percentage of following vehicles, and the density of following vehicles. The results showed that the average travel speed decreased, the follower percentage increased, and the follower density increased as the hourly traffic volumes increased. Moreover, the road performance decreased on the road covered with compacted snow compared with the dry road. The installation of an auxiliary lane at certain intervals was also found to improve the level of service for two-lane highways on both the dry road and the road with compacted snow.

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