Geometric Design Guidelines for Major Intersection Alternatives to Accommodate Multimodal Users
Research Problem Statement
Intersections on multilane arterials are becoming increasingly congested throughout the U.S. and other countries. Engineers have few good options to improve these intersections. Turn lanes, actuated signals, and signal systems have usually been employed for years. Widening and structures can be very expensive and environmentally disruptive. Transit, demand management, and intelligent transportation systems are typically years away from making a meaningful impact on congestion.
In recent years, engineers have begun employing alternatives to conventional intersections as a way to reduce congestion without great expense or other large impacts. Michigan has used the median u-turn design extensively for years, while New Jersey has used the jughandle design. New York and Maryland have successfully employed the continuous flow intersection, while Maryland has also used the superstreet design. Research has shown that there are other designs that could boost efficiency with modest extra cost or other impact, including the quadrant roadway intersection.
The American Association of State Highway and Transportation Officials’ (AASHTO) A Policy on Geometric Design of Highways and Streets contains guidelines on the design of standard intersections and guidance on the median u-turn and jughandle alternatives. The guidance provided on the design of median u-turns and jughandles, however, is limited. There is no guidance from AASHTO, in the Policy or elsewhere, on the other alternatives mentioned above. This lack of guidance is likely discouraging engineers from considering one or more of the alternatives in situations where they may be appropriate. Standard guidelines and use of these designs may lead to a decrease in delay and collisions at intersections.
Literature Search Summary
Most of the previous research on major intersection alternatives has concentrated on travel time and delay for the alternatives in comparison to each other and to conventional designs. A few papers have provided collision frequencies and rates for some of the alternatives. However, there is practically no literature providing guidance on the details of the designs.
Two recent efforts have summarized the available literature on the alternative designs. The first effort was by the FHWA (“Signalized Intersections: Informational Guide, FHWA-HRT-04-091, dated August 2004, available at www.tfhrc.gov/safety/pubs/04091/). The second effort was by Reid (“Unconventional Arterial Intersection Design, Management and Operation Strategies,” dated September 2003, available at www.pbworld.com/library/fellowship/reid/). Both efforts brought together the past findings on travel time and delay with the relatively sparse past finding on safety. The FHWA material was included in a larger document providing information on many different aspects of signalized intersection design and operation, and thus places the major alternatives in that context. Reid’s effort was more focused on the major alternatives, and he summarizes the literature related to several more alternatives than the FHWA effort. Of the five major alternatives that have been applied most often in the U.S. and/or have the most potential for travel time savings (median u-turn, jughandle, superstreet, continuous flow intersection, and quadrant roadway intersection), both of these thorough recent reviews provide a fine foundation from which this research can build.
The objective of this research is to provide guidance on the geometric and traffic control details of the major intersection alternatives, including answers to questions such as:
· Where should median openings be located?
· Where are the best crosswalk locations?
· What are the best median and island treatments?
· What sign designs best convey needed guidance information to unfamiliar drivers?
The research should include a review of previous research, two thorough recent reviews have been performed and this research can build upon that foundation. The main effort here will be an examination and evaluation of current practices. The researchers will likely need to visit and observe operations at the existing sites where alternatives have been employed. It will probably not be possible to conduct controlled experiments to evaluate the design choices, but the researchers should still be able to collect and analyze data from actual installations pertaining to some of those choices. The researchers may be able to utilize simulations and visualizations to analyze some of the design choices. Focus groups and expert panels of road users and professionals may also be excellent tools in these evaluations. The researchers must consider all expected users of intersections, including pedestrians, bicyclists, trucks, buses, users with disabilities and others. The final report should include proposed changes to AASHTO Policy as well as recommendations for changes in other standard documents such as state design manuals. The final report should also provide strategies for how to address important questions on which the quantitative evidence is currently weak.
Much of the experience with major alternatives has been outside the U.S., particularly in Mexico with the continuous flow intersection. Thus, the research effort should include visits and observations of these applications outside the U.S. Projecting how well those international experiences apply to U.S. conditions will be a critical element of the research. It should also be noted that, except where they appear as part of a larger overall scheme (as in the “Bowtie” design), roundabout design and operation are out of the scope of this project. Issues related to roundabout design and operations have been and will be addressed in other research projects.
Estimate of Problem Funding and Research Period
Urgency, Payoff Potential, and Implementation
This research topic was selected by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their combined meeting in June 2004 as being among the top 15 highest priorities for geometric design research. The problem of congested intersections on multilane arterials is serious in the United States and internationally, and it is getting worse with each year. The research is needed because, besides the designs to be investigated in this project, there are not many good alternatives for efficient and safe ways to improve at-grade intersections. However, many transportation agencies will not use these designs without the guidelines to be supplied during this project. Once the guidelines are distributed to transportation agencies and, perhaps, adopted by AASHTO in some appropriate form, designers should begin earnestly considering all options for intersection improvements.