Time-based calibration of the inertial operating speed to develop a new global consistency model

Road crashes are mainly caused by three concurrent factors: infrastructure, vehicle, and human factors. The interaction between infrastructure and human factor leads to the concept of geometric design consistency. Recently, a global consistency model was developed based on the difference between the inertial operating speed profile and the operating speed profile. The first one was defined as the weighted average speed of the previous road section based on distance and represents drivers’ expectancies, whereas the second one represents road behavior. However, drivers’ expectancies are related to Short-Term Memory which is gradually in decline and depends on time. Thus, a time-based inertial operating speed would allow a more accurate estimation of the phenomenon. This research analyzes different periods of time and weighting distributions to identify how drivers’ expectancies should be estimated. A set of 71 homogeneous road segments located in Italy were considered in the study. As a result, 25 seconds and a convex parabolic distribution should be used to calculate the inertial operating speed profile. This new way to estimate drivers’ expectancies presented better results than those obtained based on distance. Finally, the proposed consistency model was compared with the previous ones. As a conclusion, this model could assess more accurately the geometric design consistency. Therefore, the proposed consistency model is a useful tool for engineers to estimate the number of crashes and incorporate road safety to the geometric design of both new two-lane rural roads and improvements of existing highways.