Guidelines for Free Right-turn Lanes at Unsignalized Intersections on Rural Two-lane Highways

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Department of Civil Engineering, College of Engineering and Technology, 1995 - Roads - 111 pages
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Free right-turn (FRT) lanes are turning roadways at intersections that provide for free-flowing right-turn movements. The objective of the research presented in this report was to develop guidelines for FRT lanes at unsignalized intersections on rural two-lane highways in Nebraska. The development of the guidelines was based on a benefit-cost analysis, which determined the right-turn volumes required to justify the construction and maintenance of FRT lanes at these locations. The research involved the evaluation of the operational and safety effects of FRT lanes. The operational effects of FRT lanes were analyzed using kinematic and traffic flow models. Accident data analysis, field studies, and computer simulation of truck dynamics were used to evaluate the safety effects of FRT lanes. The results of these analyses were also used to determine acceptable design criteria for FRT lanes. A survey of other state agencies was conducted to identify any existing guidelines, design criteria, and experience that might be useful in developing the guidelines and evaluating the design criteria. Also, the concerns of citizens opposed to the removal of FRT lanes were examined in an effort to develop guidelines that might effectively address these concerns. The results of the research indicate that design-year right-turn AADTs ranging from 440 to 825 vehicles per day, depending on the percentage of trucks, are required to warrant a FRT lane at an unsignalized intersection of rural two-lane highways. A design speed of 40 mph was found to be a realistic design speed and the most cost-effective design speed for FRT lanes. However, design speeds up to 55 mph do not significantly reduce the cost effectiveness of a FRT lane. The AASHTO design criteria for curves on open highways were found to be appropriate for designing the curvature of FRT lanes. The AASHTO criteria for minimum-radii intersection curves were not acceptable. Also, acceleration lanes were found to improve the safety of the merging operations associated with FRT lanes. The results of the accident data analysis do not support the perceptions of concerned citizens regarding the safety benefits of FRT lanes. FRT lanes were not found to affect the frequency, severity, or types of accidents that occur at unsignalized intersections on rural two-lane highways. Thus, FRT lanes should not be perceived, or promoted, as traffic safety improvements. The benefits of FRT lanes are limited to improving the efficiency of right-turn movements.

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