Use of Narrow Lanes and Narrow Shoulders on Freeways: A Primer on Experiences, Current Practice, and Implementation Considerations

Chapter 1. Overview

This primer is designed for policy makers, transportation agency managers, designers and operators working to find solutions to today's transportation and mobility challenges. The purpose is to provide information on the use of narrow lanes and narrow shoulders to improve capacity within an existing roadway footprint, and to give the reader a starting point for exploring narrow lanes and shoulders as a potential solution.

Much of the information in this primer is presented in the broader context of Performance-Based Practical Design (PBPD). Per the Federal Highway Administration (FHWA) website (Reference 12):

PBPD is a decision making approach that helps agencies better manage transportation investments and serve system-level needs and performance priorities with limited resources. PBPD uses appropriate performance-analysis tools, considers both short and long term project and system goals while addressing project purpose and need.

The PBPD approach encourages designers and decision makers to exercise engineering judgment in identifying and analyzing alternatives — including narrow lanes — to deliver cost-effective operational improvements that meet both project and system objectives.

Topics covered in the primer include the following:

Examples of narrow lane / narrow shoulder applications – Chapter 1
Examining potential narrow lane / narrow shoulder solutions in the context of PBPD; and the role of Transportation Systems Management and Operations (TSMO) in supporting narrow lane applications – Chapter 2
Case studies of the use of narrow lanes – Chapter 3
Issues and approaches for analyzing the operational and safety impacts of narrow lanes and narrow shoulders – Chapter 4

This concept of adding a travel lane within the existing roadway footprint (by narrowing existing lanes and/or shoulders) to increase capacity at a relative low cost is not a new concept. As noted in a 1978 research document (1):

"When the congestion becomes so extensive and repetitive, measures to increase capacity or reduce demand should be undertaken. However, sufficient funds to make major changes to urban freeways may not be available, and in some instances, space may be so limited as to rule out normal expansions in roadway width. One approach that many transportation agencies are considering is the downscoping of design standards to achieve greater capacity at lower cost. The usual method to accomplish this is to reduce lane widths and to reduce or eliminate the roadway shoulders and create an additional lane for travel."

Potential scenarios for implementing narrow lanes¹ include the following (with example lane configurations and widths resulting from the implementation of narrow lanes shown in Figure 1):

Adding a general purpose lane to increase capacity and reduce recurring congestion. This can be for an extended section of roadway, or for a relatively short area as part of bottleneck reduction or to maintain lane continuity² . Examples of this approach are listed in Table 1.
Adding a managed lane, such a High Occupancy Vehicle (HOV) or HOT lane. Examples of this approach are listed in Table 2.
Adding a lane in and/or within the vicinity of an interchange, to provide additional capacity on a ramp, an auxiliary lane between closely-spaced interchanges, or additional capacity beyond the interchange to prevent traffic from backing up into the interchange area. Examples of this approach are listed in Table 3.

A related application that can involve narrow lanes and narrow shoulders is to open either the left or right shoulder — as is, or perhaps widened (with a corresponding narrowing of general purpose lanes) — to traffic during selected times of the day or when congestion warrants. The shoulder may be open to all vehicles, only light-duty vehicles, or buses only. By definition, during times of shoulder use, there is no shoulder available for vehicle refuge. Additional information on part time shoulder use, including several locations where this strategy has been implemented, is provided in Reference 6 ("Use of Freeway Shoulders for Travel – Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy; Publication No. FHWA-HOP-15-023").

Table 1. Examples: Narrowing Lanes to Add a General Purpose Lane.
Location	Route(s) and Length	Date	Cross-Sections	Reference(s)
Houston, Texas	US 59 (Southwest Freeway) Approx. 3.1 miles (both directions)	1976	4 lanes converted to 5 lanes 3 lanes converted to 4 lanes (Refer to Figure 1)	1 (Additional information provided in Appendix A)
Honolulu, Hawaii	H1 Approx. 3.5 miles(both directions)	2014	3 lanes converted to 4 lanes (11.5–12 ft. lanes re-striped to 10 ft. with reduced shoulder widths)	2
Northern Virginia	I-395 (Approx. 1.5 miles NB; 2.5 miles SB)	1989	3 lanes converted to 4 lanes (12 foot lanes re-striped to 11 ft., with reduced shoulder widths (2 ft. or less inside; 4 to 10 ft. outside)	3
Milwaukee, Wisconsin	I-94 Less than 1-mile (both directions)	2015 Planning	4 lanes to be narrowed from 12 ft. to 11 ft., with narrow shoulders, due to R.O.W constraints, as part of a reconstruction project	6 (Refer to case study herein)

Table 2. Examples: Narrowing Lanes to Add a Managed Lane.
Location	Route(s) and Length	Date	Cross-Sections	Reference(s)
Los Angeles, California	Multiple routes Approximately 49 miles	1993	5 lanes converted to 6 lanes 4 lanes converted to 5 lanes Additional lane used as HOV in nearly all cases (Refer to Figure 1)	1, 3, 4, 5, 16 (Additional information provided in Appendix)
Miami – Dade, Florida	I-95 and SR 826	Initial segment in 2008 Ongoing	4 general purposed lanes + HOV lane converted to 4 lanes + 2HOT lanes (Refer to Figure 1)	9 (Refer to case study herein)

Table 3. Examples: Narrowing Lanes to Add a Lane in the Vicinity of an Interchange.
Location	Route(s) and Length	Date	Cross-Sections	Reference(s)
Los Angeles, California	NB SR 110 connector to NB I-5	2010	Connector ramp re-striped to provide two lanes, the second lane being the shoulder for part time use. Signage installed on SR 110 to allow through and exit movements from a lane when connector ramp shoulder open to traffic.	7 (Refer to case study herein)
Everett, Washingtona	US 2 from the I-5 / US 2 Interchange EB to SR-204 (Approx. 1.6 miles)	2009	2 general purpose lanes narrowed from 12 ft. to 11 ft., inside shoulder narrowed from 4 ft. to 2 ft., and outside shoulder widened from 10 ft. to 14 ft. (all via restriping). Shoulder opened to traffic during PM peak to prevent exiting traffic (from I-5 to US 2) from backing onto I-5, and to reduce crashes in the interchange. (Refer to Figure 1)	10 (Refer to case study herein)

Note: Three general purpose lanes converted to four general purpose lanes US 59 — Houston, Texas (Reference 1).

Note: Four general purpose lanes converted to five general purpose lanes US 59 — Houston, Texas (Reference 1).

Note: Four general purpose lanes and one High Occupancy Vehicle lane converted to four general purpose lanes and two High Occupancy Toll lanes Miami, Florida Typical (Reference 5).

Note: Four general purpose lanes converted to four general purpose lanes and an High Occupancy Vehicle lane Los Angeles, California Typical (Reference 5).

Note: Re-striping of general purpose lanes and inside shoulder to accommodate part time use of the outside shoulder Washington State US 2 (Reference 10).

Note: Four lanes total (two lanes in each direction) converted to five lanes total Inner Ring Expressway, Shanghai, China (Reference 11).
Figure 1. Diagrams. Example Narrow Lanes and Shoulder Configurations.

¹The additional lane and associated capacity may operate at all times or part time (e.g., peak periods).

²Per the 3rd Edition of the AASHTO Highway Safety Design and Operations Guide, lane continuity allows a driver to remain in the through movement without changing lanes. Lack of lane continuity is usually the result of a change in the number of lanes, such as dropping a left lane at an exit without the use of an auxiliary lane.

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