With the objective of exploring whether AASHTO's existing "policy on geometric design of highways and streets" provides adequate dynamic safety of driving for new designs, redesigns, and rehabilitation strategies at curved sites, side friction factors on curved sections of two-lane rural highways were investigated. The study was based on geometric design, operating speed, and accident data for 197 curved roadway sections in new york state. To achieve this objective, a comparative analysis of side friction demand versus side friction assumed was carried out. With respect to the independent variable degree of curve, it was determined that (a) friction increases as degree of curve increases; (b) side friction assumed is higher than side friction demand on curves up to about 6.5 Deg; (c) for curves greater than 6.5 Deg, side friction demand is higher than side friction assumed; and (d) the gap between friction assumed and demand increases with increasing degree of curve. With respect to the independent variable operating speed, it was determined that (a) friction decreases as operating speed increases; (b) side friction assumed is lower than side friction demand up to operating speeds of 50 mph; (c) the gap between side friction assumed and demand increases with decreasing operating speeds; and (d) for operating speeds greater than 50 mph, side friction assumed is higher than side friction demand. With respect to the independent variable accident rate, it was determined that (a) side friction demand begins to exceed side friction assumed when the accident rate is about six or seven accidents per million vehicle-miles and (b) the gap between side friction assumed and demand increases with increasing accident rates. In general, analyses indicated that, especially in the lower design speed classes, which are combined with higher maximum allowable degree of curve classes, there exists the possibility that (a) friction demand exceeds friction assumed and (b) a high accident risk results, because at lower design speed levels the danger exists that design speeds and operating speeds are not well balanced. Thus, it is apparent that driving dynamic safety aspects have an important impact on geometric design, operating speed, and accident experience on curved roadway sections of two-lane rural highways.
Samenvatting