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Evaluating Tradeoffs of Freeway Lane Widths and Shoulder Widths


Considering the growing problem of freeway bottlenecks along with freeway demand, the possibilities of gaining significant capacity in the short term are few (other than simply widening by adding 12-foot lanes). Some cost-effective solutions include: allowing shoulder use, ramp metering with possible integrated freeway management, and adding a lane by reducing existing lane and/or shoulder width. For the latter treatment with lane narrowing, limited research has been conducted and limited guidance is available in both operations and safety. Federal Highway Administration (FHWA) is currently conducting research in operations to update and enhance the Highway Capacity Manual (HCM) analysis methodology, and to develop a new driver model or guidance on calibration in micro-simulation models. Considering that safety impacts have been mixed, the latest Texas Transportation Institute (TTI) research (Report 0-6811-1) has advanced the state of the analysis with crash prediction models and a tool to support a neutral impact of safety. The TTI report collected limited data in the state of Texas and for lane widths equal and greater than 11 feet. The recent FHWA report has located some sites with 10-foot and 10.5-foot lanes. As the technology and advances in the development of connected and autonomous vehicles increase, to what extent may narrower lanes be used for these vehicles and/or traditional vehicles without adverse impacts to throughput or safety of the traffic flow.

This study is to conduct research that is more encompassing and to develop a safety analysis methodology and tool that could be incorporated in the Highway Safety Manual (HSM). Moreover, maintenance, drainage and conversion should be studied to provide guidance or recommendations for conversion considerations. Early concerns with drainage include possible spread impacts, especially those roadways where traffic may be brought closer to a roadside barrier due to narrow shoulders. In the case of maintenance, short-term and long-term impacts should be considered. To what extent will maintenance activities for items such as snow removal and storage, cleaning activities (such as for drainage structures) or items such as bulb replacement be impacted. For longer term activities, will the narrower lanes and shoulders cause a reduction in capacity during construction for setups such as lane shifts or counter-flow lanes?


The objectives of this research are as follows:

· Enhance the safety analysis of narrowing lanes and shoulders to add a lane on a freeway.

· Develop a safety analysis tool, and material that would be appropriate for the HSM.

· Develop guidance for conversion, maintenance, and drainage considerations.

· Recommend best practice to minimize safety impacts for the flare at the beginning (to add a lane) and the taper at the termination of lane narrowing (to remove a lane).

· Develop geometric design guidance for conversion at on-ramp merge and off-ramp diverge areas.


This study is expected to develop safety evaluation methodologies, tools, and guidance for the consideration and conversion to narrow lanes. Concurrently, FHWA in a complementary effort is developing evaluation methodologies and guidance from the operations perspective.

Related Research:

· Reducing Lane and Shoulder Width to Permit an Additional Lane on a Freeway: Technical Report, Dixon K, Fitzpatrick K., et al., TxDOT Report 0-6811-1, 2015.

· Use of Narrow Lanes and Narrow Shoulders on Freeways, FHWA Draft Report, 2016, ongoing

· Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways, FHWA-HRT-05-001, 2005.

· Use of Shoulder and Narrow Lanes to Increase Freeway Capacity, NCHRP Report 369, 1995.


·Target Audience: Traffic and geometric design professionals responsible for freeways.

· Key Decision Makers: State DOT Chief Engineers.

· AASHTO Committees: The AASHTO Standing Committee on Highway Traffic Safety, Standing Committee on Highways, and Subcommittee on Design could facilitate the implementation of the research results along with Technical Committee on Geometric Design.

· Early Adopters: The research results will be easily implementable although various state DOTs who could serve as early adopters to begin using the results. Candidate DOTs for early implementation of the results include Caltrans and TxDOT. Webinars could be used to help disseminate the research results. There are no known barriers to implementation of the research products.


This research is highly recommended as most of our urban freeways are already congested or becoming congested. Lane and shoulder narrowing to add a lane is a very cost effective remedy to relieve recurring bottlenecks. These bottlenecks occur along limited freeway segments where lane narrowing can provide a quick remedy that can last until the advent of connected automated vehicles. Therefore, users need guidance on safety and operational analysis that are currently lacking.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:High
RNS Developer:Joe Bared, David McDonald, Kay Fitzpatrick, Karen Dixon
Source Info:Developed as part of the 2016 mid-year meeting of the TRB Committee on Geometric Design (AFB10), TRB Committee on Operational Effects of Geometrics (AHB65), and AASHTO Technical Committee on Geometric Design.
Date Posted:10/26/2016
Date Modified:11/14/2016
Index Terms:Highway capacity, Bottlenecks, Road shoulders, Width, Highway traffic control, Traffic safety, Highway Capacity Manual, Highway Safety Manual, Highway operations,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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