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21st Century Operations Using 21st Century Technologies
Table of Contents

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

Chapter 5. Summary and Conclusions

Implementing narrow lanes and shoulders to add a lane within an existing roadway footprint can be a viable and cost effective approach to reduce congestion. In addition to improving mobility, narrow lanes and shoulders may also be implemented with minimal negative impacts on safety and reliability. Potential scenarios for implementing narrow lanes include the following:

  • 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.
  • Adding a managed lane, such a High Occupancy Vehicle (HOV) or High Occupancy Toll (HOT) lane.
  • 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.

Narrow lanes and shoulders should not be implemented without first conducting a thorough evaluation and analysis of their ability to meet the project purpose and needs, such as provided by a Performance-Based Practical Design (PBPD) approach. Each site and segment must be evaluated individually. Analysis considerations should include:

  • Existing conditions (e.g., number of lanes and widths, shoulder widths, horizontal and vertical site distance, lateral clearance, locations and causes of congestion, traffic volumes and mix, lane and balance, current safety issues / types and causes of crashes, pavement joints and seem patterns, distances between upstream and downstream ramps.
  • Alternative configurations of narrow lanes and shoulders. In this regard, NCHRP Report 369 (Reference 3) recommends the following priority order for narrowing: narrow to 11 foot lanes, reduction in width of the left shoulder, reduction in width of the right shoulder.
  • Length of treatment (to ensure that a bottleneck is not merely relocated to some other location)
  • Estimated changes in level of service, capacity and crash frequencies / rates (and other performance measures) using appropriate analysis tools
  • Potential Transportation System management and Operations (TSMO) strategies and Intelligent Transportation Systems (ITS) technologies — in advance of and through the implementation area — to improve the operations, reliability, and safety of the narrowed lane / shoulder configuration
  • Estimated costs

This information will help in developing and getting approval of any design exceptions.

It is also important to involve emergency response personnel, enforcement personnel, and operations and maintenance staff in the analysis and evaluation. Public information and outreach to educate and alert the driving public of the changes should also be addressed.

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