Calibration of inertial consistency models on North Carolina two-lane rural roads.

Road crash occurrence is closely related to the geometric design consistency, which can be defined as how drivers' expectancies and road behavior fit. To this regard, the crash rate on a road segment increases as its consistency level decreases. To assess this phenomenon, inertial consistency models were recently developed. These models are based on the difference between the inertial operating speed, which represents drivers' expectancies, and the operating speed, which represents road behavior. The higher the difference between both speeds, the higher the likelihood of crash occurrence. This research aims to validate and calibrate these consistency models on American two-lane rural roads. For this, a total of 194 homogeneous road segments and 977 horizontal curves along 665 km in North Carolina (US) were used. As a result, the geometric design consistency was identified as a major factor of crash occurrence. The higher the difference between drivers' expectancies and road behavior, the higher the crash rate. Likewise, the greater the consistency level, the greater the percentage of horizontal curves without reported crashes. A Safety Performance Function was also calibrated to estimate the number of crashes on a road segment. Consistency thresholds were defined and tested to identify where these crashes are more likely to take place. Finally, the results obtained in this study were compared with those obtained previously on Spanish highways. To this regard, the crash rate on an American highway was 1.85 times greater than those observed on a Spanish highway under the same risk exposure and consistency conditions. Therefore, different tools were developed to enhance the assessment of road safety to the geometric design of both new two-lane rural roads and improvements of existing highways.

Modeling the impact of spatial relationships on horizontal curve safety.

The curved segments of roadways are more hazardous because of the additional centripetalforces exerted on a vehicle, driver expectations, and other factors. The safety of a curve is dependent on various factors, most notably by geometric factors, but the location of a curve in relation to other curves is also thought to influence the safety of those curves because of a driver's expectation to encounter additional curves. The link between an individual curve's geometric characteristics and its safety performance has been established, but spatial considerations are typically not included in a safety analysis. The spatial considerations included in this research consisted of four components: distance to adjacent curves, direction of turn of the adjacent curves, and radius and length of the adjacent curves. The primary objective of this paper is to quantify the spatial relationship between adjacent horizontal curves and horizontal curve safety using a crash modification factor. Doing so enables a safety professional to more accurately estimate safety to allocate funding to reduce or prevent future collisions and more efficiently design new roadway sections to minimize crash risk where there will be a series of curves along a route. The most important finding from this research is the statistical significance of spatial considerations for the prediction of horizontal curve safety. The distances to adjacent curves were found to be a reliable predictor of observed collisions. This research recommends a model which utilizes spatial considerations for horizontal curve safety prediction in addition to current Highway Safety Manual prediction capabilities using individual curve geometric features.