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1 S U M M A R Y Evaluation of the 13 Controlling Criteria for Geometric Design The FHWA has, since 1985, designated 13 specific design elements as controlling criteria for roadway design. These are design speed, lane width, shoulder width, bridge width, structural capacity, horizontal alignment, vertical alignment, grade, stopping sight distance, cross slope, superelevation, vertical clearance, and horizontal clearance. Research since 1985, culminating in publication of the most recent Highway Capacity Manual in 2010 and the AASHTO High- way Safety Manual in 2011, has developed much greater knowledge of the traffic operational and safety effects of the controlling criteria than was available when they were established in 1985. The objective of the research herein was to describe the impact of the controlling roadway design criteria on safety and operations for various urban and rural roadway types. The research team considered how the current controlling criteria or possible modified criteria (adding, dropping, or combining particular design elements), through the design exception process, influence the provision of flexibility of the design process. The scope of the research was limited to roadway design criteria and, as specified in the NCHRP Project 17-53 state- ment, did not address intersection design, roadside design, or access control. The NCHRP Project 17-53 scope included new construction and reconstruction projects, but did not include resurfacing, restoration, and rehabilitation projects. NCHRP Report 783 presents a comprehensive catalog of the known traffic operational and safety effects of the 13 controlling criteria for four roadway types: rural two-lane highways, rural multilane highways, urban and suburban arterials, and freeways. The discussion of each of the 13 controlling criteria addresses current design criteria, particularly those in the AASHTO Green Book; traffic operational effects; traffic safety effects; and mitigation strate- gies that may be considered when a design exception to the controlling criteria is approved. The report reviews highway agency experience with the design exception process and the specific controlling criteria for which design exceptions are most frequently sought. The report presents the results of data analyses conducted as part of this research that have provided new knowledge concerning the traffic operational and safety effects of the 13 controlling criteria. The results that were obtained include the following: ⢠Analysis of traffic crash data for bridges on two-lane rural highways as part of this research revealed no evidence of increased crash frequencies or severities for bridges with road- way widths (lane width plus shoulder width) narrower than the roadway width on the approach roadway. ⢠Analysis of crash data for rural two-lane highways as part of this research showed no increase of crash frequencies by crash severity level on crest vertical curves as a function of stopping sight distance for a range of stopping sight distance levels above and below the AASHTO stopping sight distance criteria. Crash frequencies increased on a crest vertical
2curve only when a horizontal curve, intersection, or driveway hidden from the view of approaching drivers by the crest vertical curve was present. ⢠Through analysis of traffic speed data collected upstream of and within horizontal curves on rural multilane highways as part of this research a model was developed to predict the reduction in traffic speed on horizontal curves, in comparison to the traffic speed upstream of the curve, as a function of curve radius. ⢠Through analysis of crash data for rural multilane highways as part of this research models were developed to predict the crash frequency by crash severity level on horizontal curves as a function of curve length and radius. ⢠Analysis of traffic speed data collected upstream and downstream of lane-width transitions on urban and suburban arterials as part of this research showed no statistically significant effect of lane width on traffic speed. ⢠Through analysis of traffic speed data collected upstream of and within horizontal curves on urban and suburban arterials as part of this research a model was developed to predict the reduction in traffic speed on horizontal curves, in comparison to the traffic speed upstream of the curve, as a function of curve radius. Sensitivity analyses were conducted to prioritize the 13 controlling criteria, by roadway type, based on their traffic operational and safety effects. Based on these priorities, specific recom- mendations on how state departments of transportation may choose to use the controlling criteria on non-Federal-aid projects were developed.
