Arterial roadways are crucial links in the national transportation system providing both local and regional mobility and access; as such, they are critical to the vitality of our economy and quality of life. As increasing demands are placed on transportation infrastructure, and freeway congestion continues to increase, arterials have become an alternative route for many drivers, resulting in the lower performance of these roadways. In many areas, arterial safety and operational performance is negatively affected by weaving maneuvers that occur at freeway interchanges and other access points where there are large turning volumes. Intensive weaving activity is often found on the arterial segment between the interchange off-ramp terminal and an adjacent signalized intersection. It is also found on frontage road segments between the freeway off-ramp and the frontage-road/cross-street intersection. Further, many alternative intersection configurations, including median U-turns and restricted crossing U-turns, weaving maneuvers can affect their operations and safety.
The extent of arterial weaving activity is directly related to the number of vehicles that enter or exit at the ramp terminal or other access point, the distance between these entry/exit points and the driveway/access point density on the arterial. The negative effect of weaving activity on traffic speed is well-documented in the context of freeway weaving areas. Although arterial weaving reduces speeds, very little is known about the magnitude of the effect especially where alternative arterial street designs are used.
Understanding the safety and operation of alternative arterial street designs and the effect of weaving on these arterials is important for state and local agencies tasked with the responsibility of maintaining these roadways. The Highway Capacity Manual (HCM) provides a methodological approach for estimating the performance of weaving areas on freeway segments. This methodology, however, is not applicable on either traditional arterial street segments or on alternative arterial streets because of differences in traffic control, traffic flow characteristic, and driver behavior between freeways and arterials. The freeway section represents uninterrupted flow with merging control for entering vehicles. In contrast, the arterial segment is often regulated by upstream and downstream signals and may use no-control, yield, stop, or signal control to regulate both the entry and the exit maneuvers. Arterial weaving capacity is significantly affected by the gaps introduced by the upstream signal and the distance available for weaving (which varies with the queue length at the downstream signal).
It is likely that the current service measures for arterial analysis, average travel speed, and service measures for freeway weaving, such as speed and density, are not appropriate for alternative arterial design and arterial weaving. Therefore, this study should seek to gain insight from the traveling population and researchers to identify appropriate measures and thresholds of performance.
The objective of the research project is to develop guidelines that can be used to evaluate the safety and operations of alternative arterial designs based on consideration of access-related weaving, signalized and unsignalized access spacing, median type, upstream and downstream intersection design and traffic control, and intermediate driveway design and control. The guidelines would assist in the identification of the best combination of arterial design and intersection design (considering both conventional and alternative intersection forms) based on consideration of operational and safety issues, access management goals, construction costs, and environmental benefits. The guidelines should explicitly address automobile, truck, pedestrian, bicycle, and transit travel modes. The guidelines should be developed to assist planners and designers with the development of new construction, reconstruction, and access management decisions. The products of this research should complement (and integrate as appropriate) the recently completed research by FHWA in the area of alternative intersections and interchanges.
A key component of this research will be the development of procedures for separately evaluating the safety and operational performance of arterial street segments. These procedures should provide a means of quantifying the effect of access-related weaving movements on safety and operations. This includes the identification, from a traveler’s perspective, of appropriate operational and safety performance measures, thresholds for the evaluation of level of service, and the development of a quantitative methodology for the estimation of those measures.
The safety evaluation procedure would be developed in a format suitable for inclusion in the next edition of the Highway Safety Manual. Similarly, the operations evaluation procedure would be developed for inclusion in the next edition of the Highway Capacity Manual. Draft text should be developed documenting recommended changes to the AASHTO document A Policy on Geometric Design of Highways and Streets (especially as it relates to Section 10.4, Access Separations and Control on the Crossroads at Interchanges).
The researchers of this project should coordinate with the researchers conducting NCHRP Project 07-23, “Access Management in the Vicinity of Interchanges.” The products of this project should leverage the data and findings from NCHRP 07-23 and extend them as needed to achieve the project objective.