Developing Design Criteria for Cost-Effective Multi-Lane Loop Ramp Design
I. Research Background
As traffic volumes in most areas continue to grow and right-of-way and available funding to build new infrastructure become limited, more emphasis is placed on adding additional capacity to existing infrastructure or constructing new facilities with higher capacities. Unfortunately, upgrading or replacing such facilities using traditional design solutions is often prohibitively costly or simply cannot be accommodated because of space availability, environmental impacts, or other factors that constrain the design environment.
Interchanges represent the most complex, and hence most challenging instance of these infrastructure needs. At these locations, the implications of difficult or innovative design choices must be carefully and thoroughly understood not only to ensure the mobility goals are achieved, but also that the priority of preserving and enhancing public safety is met. Most commonly, the capacity restriction at interchanges involve ramp connections, and the potential treatment to increase mobility service often involves (1) the construction of a multi-lane loop ramps or (2) the conversion of an existing one-lane loop ramp to a multi-lane loop ramp.
Unfortunately, taking advantage of this potentially very efficient and cost-effective solution to improving interchange capacity is extremely difficult because our knowledge base and the available guidance about the design of multi-lane loop ramp facilities is very limited. Loop ramps, as with other interchange ramp types, do have specific design and operational characteristics that must be considered as part of a ramp system (entry and exit gore areas, ramp proper, and ramp terminal intersection) to produce a safe and efficient design. However, Chapter 10 of AASHTO’s A Policy on Geometric Design of Highways and Streets (2004) (Green Book) provides no guidance on the application and design of multi-lane loop ramps
Moreover, there is little supporting research on design and operational characteristics of one-lane loop ramps let alone two-lane loop ramps. The only known research on one-lane loop ramps relates to accident (crash) experience. This research did not quantify loop ramp geometry. Consequently, there is no definitive accident data associated with one-lane loop ramp design elements. Developing a better understanding of one-lane loop ramp design and operations is an essential first step in any research program that aims to establishing design criteria for multi-lane loop ramps.
II. Research Objective
Although potential multi-lane loop ramp designs can be “pieced together” using many existing guidelines listed in Chapter 10 (for example, general guidance is given on the design of multi-lane entrance and exit terminals), this type of design does not consider the interaction between the driver, roadway and vehicle that occurs between the elements in the ramp system. This interaction is complex since any ramp system consists of multiple elements where driver workload and focus changes rapidly and the driver must cope in sequence with: the exiting roadway, exit terminal, ramp proper, entrance terminal, and entering roadway – all of which exercise a range of cognitive and physical skills.
New research on driver behavior in multi-lane roundabouts and experiences observed on existing multi-lane loop ramps can be used to research some of the perceptions about drivers traveling side-by-side on a circular section of road. However, this knowledge base is limited and often represents only analogous situations rather than the true multi-lane loop ramp situation. While we can infer findings from studies involving such facilities, a thorough understanding with respect to capacity of the loop ramps, the downstream roadway capacity past the ramp terminal, operations, safety, geometry, construction considerations, capital cost, and human factors is needed so that informed and defensible design decisions regarding the use and design of multi-lane loop ramps may be made.
The research objective is two fold. First, to establish more definitive design criteria for one-lane loop ramps incorporating the exit, ramp proper and entrance. Secondly, to provide guidance on the proper planning and location of multi-lane loop ramps and to expand the profession’s knowledge and understanding of the design of multi-lane loop ramps with respect to geometry, operations, and safety. A final report should include proposed changes to AASHTO Policy if results support a change.
Research tasks may include:
· Conduct literature search and state-of-the-art review.
· Conduct survey of US and State DOT experience.
· Identify the various types of loop ramp connections and configurations and their unique design challenges.
· Identify need for additional research and select sites for data collection.
· Identify potential micro-simulation software for calibration and subsequent use in assessing capacity and operational considerations
· Prepare interim report.
· Collect additional data at selected sites identified in Phase 1.
· Simulate various design alternatives to study the operation of one- and multi-lane loop ramps using appropriate micro-simulation software.
· Prepare a final report documenting all aspects of the research.
· Develop draft material for inclusion in AASHTO’s, A Policy on Geometric Design of Highways and Streets.
V. Literature Search
A review of the road design literature indicates that there is little application or design guidance provided on the operational and road safety impacts of 2-lane lane loop ramps. In a 1993 paper that appears to offer some observations on the use of two-lane loop ramps in North America, Walker1 reviews a number of facilities that use 2-lane loop ramps. The design advice offered at the conclusion of his paper focuses on the need to accommodate speed transitions on the entrance to such ramps and the desirability of providing dimensional generosity particularly at the merge area.
VI. Estimate of Research Funding and Period
The estimated funding for this project is $450,000. The estimated time to conduct the Phase 1, 2 and 3 research including review time is 24 months.
VII. Urgency, Payoff Potential, and Implementation
AASHTO’s, A Policy on Geometric Design of Highways and Streetsprovides minimal guidance of design parameters associated with one-lane loop ramps. In terms of multi-lane loop ramps, although general guidance is given on the design of multi-lane entrance and exit terminals, there is effectively no practical information available regarding the design of a multi-lane loop ramp proper. Additionally, no guidance is given on the proper planning and location of multi-lane loop ramps. With more detailed information, highway designers will be better able to make informed decisions regarding the applicability and design of one- and multi-lane loop ramps to various site-specific conditions.
This research is urgent due to the potential substantial savings in cost and impact (environmental, length of construction, right-of-way, collision costs, etc) that may be realized through the construction of cost-effective and efficient one- and multi-lane loop ramps versus the development of these designs based on the current completely inadequate understanding and guidance that is available. While other alternatives (i.e. adding directional or semi-directional ramps) may exist that can fulfill a role similar to that of loop ramps, the cost, property, and community impact implications of these options often prohibit their use in constrained urban areas where much of our legacy infrastructure needing upgrading currently exists. In such cases, the results of this research are urgently needed to minimize the implementation and design of one- and multi-lane loop ramp designs based on outdated information and limited knowledge that may lead to operational and safety deficiencies that are neither defensible technically nor in the best interest of the public.
(1) Walker, R. J. “Two-lane Loop Ramps: Operation and Design Considerations”. Transportation Research Record 1385. Transportation Research Board, Washington, DC. p. 133-138. 1993
(2) ITE, Freeway and Interchange Geometric Design Handbook