The Effect of New Treatments and On-Coming Traffic on Driver Performance on Combined Horizontal and Crest Vertical Curves of Two-Lane Rural Roads: A Driving Simulator Study

Undoubtedly, driver performance is one of the most important contributing factors to road safety. A great portion of crashes occurs because of drivers' misperception of road characteristics, particularly on mountainous two-lane rural roads which consist of many curves. Among the most hazardous road segments are combined horizontal and crest vertical curves which have complicated geometric features and insufficient sight distance. This study aims at investigating driver performance on combined horizontal and crest vertical curves under free flow traffic and head-on traffic conditions. In addition, three new treatments are applied to the same curves in order to compare driver performance with/without treatments and under free flow traffic and head-on traffic. The three treatments include a combination of chevron signs with the herringbones, sealed shoulder and a yellow blinking signal which is a new treatment for warning of on-coming vehicles. Fifteen kilometers of "Karaj–Chalus" two-lane rural road in Iran was replicated in a driving simulator, and the treatments and on-coming traffic were applied to some combined curves of it. Driver performance measures consisting of mean of acceleration and steering wheel angle were obtained from the simulator, and statistical tests were performed. Results indicated that combining chevrons and a warning signal resulted in lower acceleration and steering wheel angle while sealed shoulder reduced neither acceleration nor steering wheel angle. Using herring bones was found to increase only steering wheel angle. Therefore, warning signal application is suggested as the most appropriate treatment for horizontal and crest vertical curves to be applied before combined crest curves.