Effective Allocation of Available Freeway Travel Lane and Shoulder Widths
Research Problem Statement
Highway designers are under increasing pressure to maximize the use of available right-of-way in freeway corridors to provide safety, mobility, and capacity for growing traffic demand. With right-of-way limitations, increased use of context sensitive designs, and implementation of managed facilities (i.e., HOV, HOT, or TOTL), designers must maximize the use of freeway cross sections. While freeway cross section design guidance suggests that 12-foot lanes with 8- to 10-foot inside and outside shoulders is ideal, there is limited research findings on how deviations from these ideals individually, or in combination, will effect freeway operations and safety. Highway designers need guidance on the operational and safety impacts for cross section design trade-offs while trying to balance corridor capacity, project costs, public involvement, and environmental impacts.
In addition, there is concern over the part-time use of existing shoulders as HOV, HOT or general use facilities during peak hour. The trade-offs between operational benefits and safety need to be quantified. Further, the safety implications of violators using the shoulder during the off-peak period need to be quantified. Does this changed view of the shoulder as part of the drivable alignment also transfer to shoulder violation on adjacent facilities? The signing and striping of these shoulders for clear communication of the changed cross section use must also be quantified.
Shoulders are often used as the separation between special use lanes and the general purpose lanes. The impacts of providing or not providing barrier separation need to be determined. Further, when barriers are used, what shoulder widths are necessary adjacent to the barrier and what safety impacts result from these shoulder widths is a concern.
Literature Search Summary
Use of shoulder and lane widths to improve traffic operations has been researched for over 30 years going back to congested corridor projects in California and Texas (McCasland, 1978; Urbanik, 1993; NCHRP 369, 1995; Bauer et al., 2004). The research has generally shown that reductions in shoulder and lane widths can be done safely and cost effectively. The research further suggests that left should removals are preferred, but maintaining at least one shoulder is important. More specific results have often been limited by the confounding of the vast number of variables in the design environment. Results have been further difficult to obtain on research budgets that did not allow for a comprehensive experimental design.
Some research specifically related to freeway lane and shoulder widths was directed towards their effect on freeway free flow speed as defined in the Highway Capacity Manual (HCM). Free flow speed is used in the HCM to establish speed/ flow relationships and associated values for maximum flow rates, v/c ratio, and density for various levels of service. The research indicates that 12 ft. lanes and 6 ft. lateral clearance on the right are optimal. Reducing these widths has a negative effect on free flow speed and consequently a reduction in flow rate. There was no attempt to link accidents with either lane width or shoulder width. No research has been accomplished for freeway cross section investigating the safety and operational tradeoffs of the allocation of lane and shoulder width across the total cross section. This topic is very much related to context sensitive design, in particular associated with freeway widening or modification to increase capacity or to add HOV/managed lanes.
The objective of the research is to provide quantitative safety and operational outcomes for use of shoulder widths of zero to 12 feet (possible up to 14 feet for shoulders used as buffer separation for special purpose lanes) and lane widths of 10 to 12 feet (possibly up to 14 feet for special purpose lanes). The research should also quantify the impact of using these shoulder and lane widths in combination. The focus of the research should also be on existing facilities that would be rehabilitated or reconstructed. As part of this retrofit, the impact of choices of lane widths, inside and outside shoulder widths must be quantified to allow for the safest and most efficient reuse of the available cross-section width. The application of current standards to new facilities is less interesting.
Another objective of the research is to develop a tool for designers to assess the cross section design trade-offs. Existing simulation models do not properly address the issues that are requested to be investigated. Thus, a simulation model or recalibration of existing models should be accomplished based on field observations as part of this research to create a user tool for cross-section analysis.
- Conduct literature search and state-of-the-art review.
- Conduct a survey of U.S. and state DOT experience.
- Identify study/research sites and data collection methodology.
- Develop experimental design that will address the objectives of the research and not result in past research deficiencies
- Prepare an interim report
- Collect data at identified sites.
- Based on data collected develop new or calibrate existing operational/design models to use in simulating operation on various freeway cross sections.
- Prepare a final report summarizing all aspects of the research.
- Develop draft material for replacement of relevant sections of AASHTO’s, A Policy on Geometric Design of Highways and Streets.
Estimate of Research Funding and Period
Recommended Funding: $750,000
Research Period: 36 months
Urgency, Payoff Potential and Implementation
The 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 one of the ten highest priorities for research. The research is needed to provide guidance to highway designers on the trade-offs of shoulder and lane width selection in freeway corridors. The results will be used nationally in the design of freeways, HOV lanes, toll roads, and special use lanes.