A Case for Differentiating Design Consistency Evaluation between Day and Night

A large percentage of fatal crashes occur at night: 47 percent in the USA and 36 percent in the EU and the nighttime traffic death rate is 4.4 times higher than in the day. Drivers rely on visual clues to complete the driving task and at night, they rely on their headlights to illuminate the distance ahead, which on average is about 160 feet. Thus, a possible contributing factor to higher crash rates at night is the reduced sight distance as it compares to that in the day. Studies have shown that speeds are virtually the same at day and night. These issues can become problematic at horizontal curves not providing drivers with adequate time to adjust their speed to safely negotiate the curve: a process that they can complete at daylight conditions. This study is the first step towards demonstrating the influence of horizontal curvature on nighttime crashes through a preliminary analysis. The objective of the study was to identify the magnitude of the problem and provide guidance for future research. Roadway segments from USA, Greece and Italy were examined and findings indicate that the increased crash occurrence at night could be related to the curve radius. Sharper curves showed an increase in crashes and crash rate when they were compared to their corresponding daytime crashes. Design consistency was evaluated using the radii of successive curves and the data supported the presence of differences in crashes between day and night. Additional work is needed to further explore these issues.