The relationship of safety to five candidate measures of geometric design consistency for rural two-lane highway alignments was examined. The measures that were evaluated included: ( a) speed reduction on a horizontal curve relative to the preceding tangent or curve, ( b) average radius, ( c) ratio of maximum radius to minimum radius, ( d) average rate of vertical curvature, and ( e) ratio of individual curve radius to average radius. All of these measures were found to have statistically significant relationships to accident frequency in the direction expected and are recommended as candidate measures for assessment of geometric design consistency. The relationships between design consistency measures and accident frequency were developed using Poisson, negative binomial, and lognormal regression analysis.
A proposed speed profile model was used to estimate the reduction in 85th percentile speeds from the approach tangent to the midpoint of 1,126 horizontal curve sites on rural two-lane highways in three states. The sites were divided into eight speed-reduction intervals, the mean accident rate and mean speed reduction were computed for each category, and linear regression was performed to analyze the statistical relationship between mean accident rate and mean speed reduction. Similar analyses were performed with degree-of-curvature intervals to compare mean degree of curvature and mean speed reduction as predictors of accident experience. The results suggest that estimated speed reduction is a useful measure that helps explain how accident experience at horizontal curves on rural two-lane highways varies with degree of curvature. Horizontal curves that require speed reductions [generally, curves sharper than about 4°, a condition that corresponds with design speeds less than 100 km/h (60 mph) and estimated 85th percentile speeds less than drivers’ desired speeds on long tangents] have higher accident rates than curves that do not require speed reductions. When curve sites are grouped into speed-reduction intervals, there is a statistically significant relationship between the intervals’ mean accident rate and mean speed reduction. The mean accident rate increases approximately linearly with the mean speed reduction.
A procedure was developed for determining work zone speed limits usable during the design or construction phases of a roadway construction project. The procedure classifies work zones by the potential hazard present, as represented by the location of work activities or traffic controls in relation to the traveled way, instead of the prevailing speed of traffic. Research by others showed that accident rates increase with deviation from average speed. The most desirable work zone speed limit policy minimizes the increase in accident rate during the construction period. Accident and speed studies showed that the minimum increase in accident rate and speed variance occurred in work zones with a 16 km/h (10 mph) reduction in the work zone speed limit. In a follow-up study, the procedure was implemented on a trial basis in work zones with speed limits up to and including 105 km/h (65 mph). Research showed that the procedure was implementable and can be used to establish speed limits that maximize traffic safety in work zones. Guidelines were developed for implementing the procedure.
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