Evaluating Tradeoffs of Freeway Lane Widths and Shoulder Widths
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.
· 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,
· Use of Shoulder and Narrow Lanes to Increase
Freeway Capacity, NCHRP Report 369, 1995.
Audience: Traffic and geometric design professionals responsible for
Decision Makers: State DOT Chief Engineers.
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
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|
|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.|
|Index Terms:||Highway capacity, Bottlenecks, Road shoulders, Width, Highway traffic control, Traffic safety, Highway Capacity Manual, Highway Safety Manual, Highway operations, |
Operations and Traffic Management
Safety and Human Factors