Consider road geometry carefully before implementing part-time shoulder use lanes.

PTSU's effects are strongest along congested segments of road, but can be severely impacted by placement and positioning of ramps or merging lanes.

Date Posted

Operational Evaluation of Part-Time Shoulder Use for Interstate 476 in the State of Pennsylvania

Summary Information

Part-time shoulder use, or PTSU, is a capacity-increasing strategy in which the road shoulder is temporarily utilized during peak commuting periods. It sees sporadic application in the United States, predominantly for transit. As a dynamic lane management strategy that helps to address issues of limited land availability for permanent arterial expansions, it is likely to become increasingly relevant and effective as connected vehicles make more flexible management possible.

This paper consists of a literature review and a simulation-based implementation of PTSU along a stretch of I-476 in Philadelphia. Several different scenarios were tested to determine if limiting the PTSU lane to either passenger vehicles or heavy trucks would be more effective than simply being an additional general-use lane. While PTSU lanes are frequently bus-only, there are no transit routes along the selected segment.

Because of its low prevalence, some of the intricacies of PTSU implementation are not fully understood. Though this report is a simulation rather than a pilot deployment, its authors used the results of their investigation to inform a set of recommendations to guide further research into how to best add PTSU along a route.

The researchers' literature review revealed that the benefits of PTSU can vary dramatically. In one implementation, travel time was reduced to 16 percent of what it had been previously, while others more consistently found reductions of around 30 percent. The study's own simulation found travel time reductions greater than the general range of values given in the literature, further indicating that the method's effectiveness can be somewhat unpredictable.

The simulation further offered several key points for suggested implementation of PTSU:

  • If possible, PTSU should be implemented in the left shoulder, as it is generally found to be more effective. One study found that left-lane PTSU was ten times more effective at reducing travel time than right-lane PTSU.
  • The vehicle composition of the roadway should be taken into consideration, as heavy vehicles can negatively impact the effectiveness of PTSU lanes.
  • The extent of a PTSU zone should be carefully considered: Extending it too far can cause it to lose its cost-effective advantage; however, if it is not wide enough, its operational benefits can be impacted by congestion in other areas of the road.
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