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21st Century Operations Using 21st Century Technologies

Decision Support Framework and Parameters for Dynamic Part-Time Shoulder Use:
Considerations for Opening Freeway Shoulders for Travel as a Traffic Management Strategy

Chapter 1. Introduction

Part-time shoulder use (PTSU) is a transportation systems management and operations (TSMO) strategy that reduces congestion-related delay by allowing use of the left or right shoulders as travel lanes during some but not all hours of the day. PTSU is also referred to as hard shoulder running, hard shoulder use, temporary shoulder use, or a variety of other names. This strategy is primarily used in locations where congestion recurs due to lack of peak period capacity through a corridor (by time of day, and/or day of week, special events) and where other alternatives to improve operations are infeasible, cost-prohibitive, or cannot be realized within short periods of time.

As the reliability of automobiles and tires has increased and incident management strategies have improved, the need for shoulders to serve exclusively as refuge areas 24 hours per day has decreased. Many agencies in the United States already employ PTSU on freeways. In some cases, PTSU provides an interim solution until conventional widening occurs. For information on PTSU in general, refer to the Federal Highway Administration (FHWA) report Use of Freeway Shoulders for Travel — Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy, hereafter called the FHWA report on the Use of Freeway Shoulders for Travel. PTSU is most cost-effective in constrained right-of-way conditions, provided minimum geometric values (such as shoulders wide enough for vehicle travel) and pavement strength are met. PTSU is classified into three types:

  • Bus-on-shoulder (BOS) – open only to authorized buses and usually at the driver's discretion.
  • Static part-time shoulder use (S-PTSU) – open to vehicles only during predetermined hours.
  • Dynamic part-time shoulder use (D-PTSU) – open to vehicles in response to realtime traffic conditions.

Table 1 notes key advantages and challenges of S-PTSU versus no shoulder use and D-PTSU versus S-PTSU.

Table 1. Advantages and Challenges of D-PTSU.
Scenario Advantages Challenges
Static part-time shoulder use (S-PTSU) versus No Shoulder Use At and upstream of recurring congested bottlenecks: reduces congestion and potential for congestion-related crashes. Reduced space in certain hours of day for:
  • Disabled vehicles
  • Emergency response
  • Incident clearance
  • Enforcement
  • Crash investigation
  • Maintenance

Must have or provide adequate geometric widths, lateral and vertical clearances, sight distances, drainage, and pavement structural section for traffic to operate on shoulder lane.

No ability to respond to day-to-day variation in traffic conditions.

Dynamic part-time shoulder use (D-PTSU versus S-PTSU) Same advantages of S-PTSU, plus the ability to address non-recurring congestion by opening the shoulder as needed (outside of fixed time periods). Same challenges as for S-PTSU, except there is an ability to respond to day-to-day variation in traffic conditions.

Requires frequent dynamic message signs, real-time video surveillance, transportation management center staffing for opening/closing shoulder any time shoulder may be opened, excellent 24/7 communication and coordination between operations, maintenance, emergency response, and enforcement personnel

Currently in the United States, D-PTSU, the focus of this report, is less common than the other types of PTSU. D-PTSU flexibly opens shoulders for travel beyond fixed (or static) time periods, typically using either a speed-based decision parameter, a volume-based decision parameter, or some combination of the two. In some cases, there may also be "core" hours when the shoulder is always open due to recurring congestion. D-PTSU, in the appropriate circumstances, can increase facility capacity, reduce delays, and improve travel time reliability. D-PTSU can be a particularly cost-effective component of a comprehensive agency TSMO strategy for addressing congestion and reliability issues within the transportation system. D-PTSU represents an important advancement and maturity in agency TSMO practices and is consistent with active and integrated operations promoted through two key U.S. Department of Transportation programs: Active Transportation and Demand Management (ATDM) and Integrated Corridor Management. More information on these programs is available at

This report presents a decision framework and a process that agencies can use to identify the appropriate decision parameters for opening the shoulder on their D-PTSU facilities as part of the agency's traffic management strategy. Decision parameters are presented within the context of a decision support framework that considers non-operational factors as well as operational decision parameters to help an agency determine if it is appropriate to open or close the shoulder at any given time. The report further provides insights on decision parameters in the form of speed and volume thresholds that would activate opening the shoulder on a D-PTSU system. While D-PTSU could be applied to an arterial, there are no known applications to date within the United States, and this report focuses on freeway applications of D-PTSU.

Choosing Part-Time Shoulder Use

The decision to pursue PTSU should be made as part of a comprehensive assessment founded on performance-based practical design (PBPD) and TSMO options for achieving the agency's performance objectives for the facility design and operations. First, the physical feasibility of PTSU should be evaluated to determine if it is a feasible option, and a region should decide if the PTSU strategy is consistent with its long-term transportation goals and objectives. Then, a preliminary assessment should be made to identify one or more design and operations concepts for evaluation. This assessment, conducted under the overall umbrella of a PBPD process, should assess the operational and safety effects of part-time shoulder use to ensure it is indeed a cost-effective means for achieving the agency's performance objectives for the facility. Throughout this evaluation, key planning and environmental, maintenance, operations, design, and emergency responder stakeholders should be involved to ensure a successful outcome. Figure 1 shows these PTSU planning considerations.

It remains the policy of FHWA that constructing and maintaining roadway shoulders along all major and minor arterials and freeways provides inherent value. Shoulder width is one of the 10 controlling criteria that FHWA requires in a formal written design exception if minimum design criteria are not met on "high-speed" roadways on the National Highway System. Refer to 23 CFR 625.3(f) and the memorandum "Revisions to the Controlling Criteria for Design and Documentation for Design Exceptions", dated May 5, 2016, available at Aside from their structural benefits for pavement and drainage, shoulders provide refuge for vehicles in emergency situations, access for first responders, and an additional recovery area for drivers trying to avoid conflicts in the adjoining travel lanes. The safety benefits of shoulders are documented in the AASHTO Highway Safety Manual and other studies. Because of these factors, the decision to use shoulders for travel should be carefully considered and limited in both its application and period of usage.

Figure shows considerations in choosing part-time shoulder use.
Source: FHWA
Figure 1. Diagram. Considerations in choosing part-time shoulder use.

Although part-time shoulder use can be a very cost-effective solution, it may not be an appropriate strategy where minimum geometric clearances, visibility, and pavement requirements cannot be met, or where it may have an adverse impact on safety. Appendix A provides questions agencies can consider during the planning, design, implementation, and operations of PTSU facilities, and gives an insight into the broad range of topics associated with PTSU.

Relationship with Other Active Traffic Management Treatments

D-PTSU is often implemented in combination with other active traffic management (ATM) treatments. Selected ATM treatments and ways they enhance PTSU operations are noted below:

  • Ramp metering – for right-side PTSU (on a freeway with right-side ramps), metering can mitigate the conflicts between shoulder traffic and ramp traffic by preventing platoons on entrance ramps. Additionally, activating ramp meters may reduce the duration of PTSU by providing an incremental increase in freeway capacity.
  • Dynamic junction control – PTSU may create conflict points or necessitate lane-changing if implemented "through" interchanges with multilane entrance or exit ramps. To prevent this, dynamic junction control could be used to change lane assignment when the shoulder is open and reduce the number of ramp lanes and add or drop a freeway lane (general purpose or shoulder) onto a ramp.
  • Dynamic lane assignment – If sign structures and communication are constructed to indicate lane control for the shoulder (typically with a green arrow and red X), they could also be installed over all lanes to close lanes when an incident, construction, or maintenance activities dictate it. Closure of a general purpose lane could be accompanied by opening the shoulder to minimize the loss in capacity.
  • Dynamic speed limit – When the shoulder is open to traffic, it may be advantageous to lower the speed limit due to reduced lateral offset between traffic and roadside objects and potentially reduced stopping sight distance for traffic that is closer to a barrier wall. Additionally, a lower speed limit may reduce the duration of PTSU by providing an incremental increase in freeway capacity.
  • Queue warning – While PTSU reduces the likelihood of queues, it typically provides additional data collection, making queues easier to identify, in turn making it possible to warn drivers of downstream delays more reliably.

PTSU is an investment for an agency. Generally, the investment in roadway construction is less than a conventional widening project, but the investment in the transportation management center (TMC) and overall facility management may be greater. Implementing the second, third, or fourth ATM treatment is generally not as challenging as implementing the first, and there are benefits and efficiencies in implementing strategies together.

Network Considerations

Freeways supply much of the capacity in a region's road network, and changes in their capacity may have broad implications for the region's travel patterns. PTSU increases freeway capacity in some hours of the day and leaves it unchanged in other hours. This change may not have an effect on freeway volume, may shift volume from the shoulders of the peak period to peak hours themselves, or it may increase volume on the freeway to due to diversion from other facilities and/or induced demand.

The degree to which any of these changes happen will vary from freeway to freeway, and be influenced through such factors as:

  • The availability/viability of alternate roadway routes.
  • The availability/viability of multimodal travel options.
  • Driver familiarity with the region and comfort taking alternate routes and modes.
  • Area type (urban versus rural).
  • Type of trips served.
  • Length of PTSU segment
  • Location of nearby bottlenecks.

In short, many factors influence how the choices of drivers will vary when PTSU is implemented. Like any larger freeway project, a PTSU project should not be analyzed in isolation but as part of a regional network. Macroscopic tools such as travel demand models are useful for estimating the network-level effects of PTSU prior to implementation.

Purpose, Scope, and Target Audience

The purpose of this report is to provide agencies with relevant information and experience for implementing and operating D-PTSU on freeways. This report includes an overview of D-PTSU domestic and international practices and focuses on identifying the decision parameters for determining when to open and close D-PTSU. Specifically, this report assists agencies in answering the following questions:

  • Would D-PTSU be an appropriate strategy in a location where no part-time shoulder use (even static) is currently in place?
  • Should D-PTSU be considered in a location where static part-time shoulder use is in place?
  • How can the operations of an existing D-PTSU installation be optimized through careful selection of speed- and volume-based "decision parameters" that are then used on a realtime basis to decide to open the shoulder?
  • What are the considerations for closing the shoulder?
  • When, if at all, should conversion of PTSU into a permanent full-time lane be considered?

This report does not address:

  • D-PTSU on arterial streets, because of the lack of U.S. experience with arterial D-PTSU.
  • The "part-time" use of a shoulder in work zones during construction (e.g., as part of a lane shift or lane closure).
  • Static (fixed hours of day) part-time shoulder use (S-PTSU). S-PTSU is covered in the FHWA report Use of Freeway Shoulders for Travel, Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy (FHWA-HOP-15-023).
  • Pedestrian and bicycle considerations.

The target audience for this report consists of state DOTs, toll agencies, MPO planners and designers, and TMC managers and operators.

Organization of Report

The report is organized as follows:

Chapter 1. Introduction describes the purpose, scope, content, organization, defines dynamic part-time shoulder use, and identifies the intended audience for the report.

Chapter 2. Overview of Dynamic Part-Time Shoulder Use describes how D-PTSU works, intelligent transportation system infrastructure needs, the advantages of D-PTSU over S-PTSU, the relative challenges of D-PTSU compared to S-PTSU, and examples of D-PTSU installations around the world.

Chapter 3. Decision Support Framework describes the considerations for developing a concept of operations (ConOps) for dynamic part-time shoulder use. It describes the overall concept of decision parameters for opening and closing the shoulder, describes the various levels of dynamic operations for a part-time shoulder use lane, and provides a decision support framework for choosing the decision parameters and level of dynamic operations for the part-time shoulder lane. The role of non-traffic considerations such as maintenance, weather, shoulder blockages, and emergency response are discussed as well.

Chapter 4. Decision Parameters for Opening Shoulder describes reactive and predictive methods for determining when a dynamic shoulder should be opened to traffic, with a focus on traffic-related decision parameters.

Chapter 5. Considerations for Closing the Shoulder describes traffic and non-traffic considerations and decision parameters (maintenance, weather, incidents, emergency response, and safety) for closing the shoulder.

Appendix A lists questions agencies can consider during the planning, design, implementation, and operations of PTSU facilities.

Appendix B provides fact sheets for national and international examples of dynamic part-time shoulder installations.

Appendix C describes software and manual methods for developing decision parameters for opening and closing D-PTSU lanes. This appendix summarizes the Product Limit, FREEVAL, and VISSIM methods.

Appendix D provides tables with generalized thresholds for opening a shoulder on a typical freeway with default traffic characteristics.

Appendix E provides a list of additional resources on PTSU, including D-PTSU.

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