Office of Operations
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 3. Decision Support Framework for Dynamic Shoulder Use Operations

This chapter describes the considerations for developing a concept of operations (ConOps) for dynamic part-time shoulder use (D-PTSU). It describes the overall concept of decision parameters for opening and closing a freeway shoulder to traffic, describes the various levels of dynamic operations for PTSU, and provides a decision support framework for choosing the decision parameters based in part on the level of D-PTSU. Methods for selecting initial decision parameter values are provided in the next two chapters.

Systems Engineering

D-PTSU projects are ITS projects and require systems engineering if federal funds are used.

Systems engineering, in general, is an organized, interdisciplinary approach to developing and implementing a system. For Intelligent Transportation System (ITS) projects using Federal-aid highway funds, a systems engineering analysis is required per 23 CFR 940.11. D-PTSU projects, by their very nature, are ITS projects. A variety of methods exist for systems engineering, including iterative methods (e.g., Agile), and the waterfall method. The waterfall method, illustrated with a Vee diagram, is shown in figure 6 and lists activities commonly done as part of systems engineering. Developing a concept of operations is a key systems engineering activity and is where the details of D-PTSU operation are determined and documented.

Vee diagram shows systems engineering for intelligent transportation systems projects.
Source: FHWA (
Figure 6. Diagram. Systems Engineering V diagram for intelligent transportation systems projects.

This diagram captures pre-installation activities such as planning, requirement specification, and design, as well as post-installation activities such as testing, validation, operations, and maintenance. D-PTSU systems often make us of existing ITS infrastructure, add additional infrastructure for D-PTSU, and sometimes add additional infrastructure for other ATM treatments such as variable speed limits. Systems engineering provides a process for agencies to ensure the necessary components are installed and function both independently and with each other so the D-PTSU system operates efficiently and as intended. A key step in the systems engineering process is the development of a concept of operations (ConOps) document that specifies how D-PTSU will operate and "look" to drivers. The following section provides additional information on ConOps documentation.

Developing the Concept of Operations

A typical ConOps document prepared by an agency implementing D-PTSU describes the rationale for selecting D-PTSU, the objectives for the operations of D-PTSU, and how the D-PTSU will be operated from the points of view of the agency operator of the facility, maintenance personnel, emergency responders, transit operators, local agencies, and any other relevant stakeholders. The ConOps and its more detailed supporting documents and references, provide the "road map" for how the D-PTSU will be operated by the agency. A typical ConOps for a D-PTSU will contain the following sections:

  1. Executive Summary called the "Scope" that provides an overview of the contents of the ConOps.
  2. Introduction laying out the objectives for operations of D-PTSU, performance measures, and identifying partners and stakeholders.
  3. Background describing the setting, listing assumptions and constraints, and listing the resource documents used in preparation of the ConOps.
  4. Operations of the D-PTSU from the points of views of the partners/stakeholders. Scenarios may be used to help flesh out the operating parameters for each partner.
    1. Transportation management center (TMC) operations.
    2. Maintenance operations.
    3. Emergency responder operations.
    4. Transit operations.
    5. Local agency interface.
    6. Other.

Additional information on the preparation of a ConOps can be found in the Federal Highway Administration (FHWA) report: Developing and Using a Concept of Operations in Transportation Management Systems, FHWA-HOP-07-001.

The remainder of this chapter will walk readers through the key decisions unique to D-PTSU that should be made in the course of preparing the ConOps for D-PTSU. The key decisions include:

  1. The level of D-PTSU.
  2. Selection of decision parameters for opening and closing D-PTSU.
  3. Development of decision support framework for opening and closing D-PTSU.
  4. Determining if and when D-PTSU should become a conversion to a full-time lane.

The following two chapters then describe various methods that an agency might employ to select the decision parameters for opening and closing D-PTSU. These initial values can be included in the initial ConOps draft and should be refined later on as the agency gains operating experience with D-PTSU.

Candidate Part-Time Shouder Use Facilities

As discussed in chapter 1 of this report and more broadly in the 2016 FHWA report on the Use of Freeway Shoulders for Travel, PTSU—whether dynamic or static—is not appropriate for all freeways. Figure 7 lists characteristics of facilities particularly well-suited for PTSU operations.

Diagram reads from top to bottom: Congestion at recurring or non-recurring times; congestion-related crash history; constrained right-of-way; short-term need for improvements; absence of downstream bottlenecks; and regional familiarity with TSMO.
Source: FHWA
Figure 7. Diagram. Considerations in choosing part-time shoulder use.

Selecting the Level of Dynamic Part-Time Shoulder Use

As part of the development of the ConOps, the agency should conduct a self-assessment of its capabilities for operating D-PTSU. The ConOps should document this self-assessment and provide the rationale for the selected level of D-PTSU to be implemented.

When it comes to opening and closing the shoulder on D-PTSU facilities, agency practices fall within a spectrum of options rather than a single operational model. This spectrum encompasses unique attributes of traffic patterns, driver populations, and agency capabilities from facility to facility. The levels presented in table 6 can help agencies understand what characteristics their PTSU facilities may initially have and what characteristics they may eventually have as they mature. The levels do not represent rigid categories, but are intended to capture the range of existing PTSU practices or those that can be planned. As agencies move to a higher level, better operations results are expected, but each higher level also requires more resource, staffing, and funding commitment by the agency. Generally speaking, agencies opening their first PTSU facility may want to start with level 1 or 2 operations and progress to higher levels over time as they become more comfortable and skilled with D-PTSU operation. In general, higher levels are more beneficial over the long term, although on shorter facilities it is possible that the benefits of a higher level of PTSU do not outweigh the costs.

Table 6. Levels of part-time shoulder use.
Level Title Description
0 No part-time shoulder use (PTSU) Shoulder is never opened to traffic.
1 Static PTSU Shoulder is only opened to traffic at predictable, fixed hours of day and days of week.
2 Dynamic PTSU with core hours and scheduled variation Shoulder is opened to traffic during recurring "core" hours and days of the week and may also be opened outside of those core hours in a scheduled, pre-determined manner for special events or seasonal variations.
3 Dynamic PTSU with core hours and unscheduled variation Shoulder is opened to traffic during recurring "core" hours and days of the week and may also be opened outside of those core hours in response to realtime or anticipated traffic conditions.
4 Fully Dynamic PTSU Shoulder is opened and closed purely in response to or in anticipation of factors such as traffic congestion, demand surges, events, incidents, weather, maintenance needs, incident management needs, or enforcement needs. There are no "core" hours and days of the week when the shoulder is always opened regardless of traffic conditions.

For any level of PTSU (except for level 1 with static signs), a shoulder may be closed during "core" hours of operations for many reasons; for example, if the shoulder is physically blocked (e.g., by a disabled vehicle), if an agency deems it is unsafe to open the shoulder (e.g., during a snowstorm), or if another stakeholder requests it (e.g., law enforcement in response to an incident in adjacent general purpose lanes). Further discussion on each level follows.

Level 1 (Static PTSU) can serve as a precursor to dynamic PTSU. Some agencies have operated a static PTSU facility for the first few months of operations and then moved to dynamic operations as they and other stakeholders became more comfortable with PTSU. S-PTSU needs the same physical roadway conditions needed for D-PTSU, such as, for example, shoulder width, pavement quality, pavement markings, or offsets to fixed objects.

Level 2 (Dynamic PTSU with core hours and scheduled variation) is similar to S-PTSU on most days of the year. Variations to core hours of operations are known and communicated in advance to stakeholders such as law enforcement and emergency responders. For example, with Level 2 D-PTSU, the shoulder may be opened on weekends or earlier on a Friday afternoon than other weekday afternoons due to a special event. Such an opening/closing schedule could be predetermined and could be reflected in interagency agreements, memorandums of understanding, or legal statutes that authorize PTSU at certain times.

Level 3 (Dynamic PTSU with core hours and unscheduled variation) is more responsive to traffic conditions than Level 2 D-PTSU. In a Level 3 deployment, the shoulder is opened on an ad-hoc basis if traffic conditions merit. Core hours of operations would remain so that the shoulder is always open regardless of traffic conditions (unless it is physically blocked, or an agency deems it is unsafe to open the shoulder). Users of the shoulder (e.g., maintenance workers, emergency responders, and law enforcement) should be aware that they will need to adapt to the shoulder being open on short notice at any given time. Operating agencies will need appropriate TMC and incident management personal in place at all hours.

Level 4 (Fully Dynamic PTSU) deployments open the shoulder in response to realtime and projected traffic conditions alone and do not maintain core hours of operations. Historical volumes, speeds, and other data may influence when an agency chooses to open and close the shoulder, but there is no "commitment" or "expectation" to open during certain hours if conditions on a given day do not merit it. Many European agencies, including the Hessen state in Germany, use Level 4 D-PTSU. Level 4 D-PTSU is also well suited for freeways with non-recurring congestion such as rural intercity routes and rural recreational routes where core hours of operations could not be determined.

These levels of D-PTSU are somewhat analogous to the Capability Maturity Model (CMM) used to characterize a region or an agency's abilities and experience with transportation systems management and operations (TSMO). It is generally easier for an agency with PTSU experience to implement Level 4 D-PTSU than it is for an agency without PTSU experience. The D-PTSU levels and CMM levels are not intended to correspond to one another (i.e., it is not implied that Level 3 D-PTSU is most appropriate for a CMM Level 3 agency).

Selecting Shoulder Operations Decision Parameters

One of the key pieces of information to be included in the ConOps is the set of performance measures and thresholds that will be initially used to determine when to open and close the shoulder to traffic. These initial values may then be refined later as the agency operator gains experience with the D-PTSU and as demand patterns evolve on the facility.

Full shoulders provide safety benefits for the traveling public on a freeway facility. Shoulders provide a place for disabled vehicles, enforcement, maintenance, and temporary snow storage that is away from the travel lanes. Shoulders should not be used for travel unless the benefits (safety and delay improvements) of opening the shoulder to travel exceed those of keeping the shoulder closed.

Breakdown is the transition from noncongested to congested conditions typically observed as a speed drop accompanied by queue formation.

Research and international operating experience (see chapter 2) suggest that shortly before the beginning of traffic congestion (breakdown of smooth traffic flow on the freeway) is an appropriate point for dynamically opening a shoulder to travel. "Breakdown," as defined in the Highway Capacity Manual, is "The transition from noncongested conditions to congested conditions typically observed as a speed drop accompanied by queue formation." (TRB 2016) The appropriate point for closing the shoulder is when it will not cause congestion in the full-time lanes of the freeway. This is the basic philosophy behind establishing decision parameters for when the shoulder should be dynamically opened and closed. Chapters 4 and 5 cover specific traffic operations decision parameters for opening and closing the shoulder in greater detail.

Other considerations related to policies or physical condition of the roadway may override the traffic operations decision parameters. For example:

  • Obstructions on the shoulder such as debris, snow, ice, disabled vehicles, enforcement personnel, and maintenance equipment may prevent or delay the agency from opening the shoulder lane to traffic.
  • Agreements with emergency responders or law enforcement may limit times that the shoulder can be opened.
  • Legislation may place statutory limits on when and/or how frequently the shoulder may be opened.
  • Opening the shoulder typically requires one or more dedicated TMC staff to be present, and these personnel may not be available 24 hours a day.
  • Environmental approval of projects may be contingent upon the shoulder remaining closed at some times to minimize air quality, noise impacts, and associated mitigations.

Nationally and internationally, the process of opening and closing the shoulder has historically been mostly up to the discretion and experience of operators in the TMC. Some TMC operators will have predictive algorithms or control systems to support their decisionmaking process in the transportation management center, while others make decisions without software by reviewing incoming data. On level 3 and 4 D-PTSU facilities, operators have historically opened the shoulder lane based on the thresholds of traffic volumes, speeds, or core hours of operations prior to the expected breakdown.

Interviews with agency operators indicated they currently use one or more of the following traffic operations decision parameters to determine when to open the shoulder lane to travel:

  • Traffic volume greater than or equal to 1,400–1,500 vehicles per hour per lane (veh/h/ln).
  • Vehicle speeds less than or equal to 40-55 mi/h. These speeds are different in the United States, Europe, and South Korea and change from facility to facility.
  • Consistent peak periods from historical traffic volume information.
  • Special events.
  • Congestion caused from crashes.
  • Operator's discretion, based on experience.

Agencies generally use one or more of the following decision parameters to determine when they should close the shoulder to travel:

  • Traffic volume less than 1,400–1,500 veh/h/ln (if shoulder were to be closed).
  • Vehicle speeds greater than 40–55 mi/h.
  • After the peak periods.
  • Crashes and obstructions on the shoulder.
  • Disabled vehicles.
  • Extreme roadway and weather conditions (the shoulder is not opened or closed if open).
  • Operator's discretion, based on experience.

As shown in figure 8, an agency may use several metrics for predicting traffic breakdown and therefore determining when to open the shoulder to travel. Metrics include historical data on when congestion has occurred in the past, traffic model predictions of congestion (breakdown), best practices from other agencies operating similar facilities, as well as conditions and needs specific to the local facility.

Flowchart starts with Uncongested Conditions, moves to PREDICT Breakdown, moves to Open Shoulder, and ends in Dissipate Congestion. PREDICT Breakdown branches out to Real-Time data, predictive models, best practices, and facility-specific needs. Open Shoulder has an astrisk.
* Assuming there are no issues with maintenance, law enforcement, environmental conditions. etc.
Source: FHWA

Figure 8. Diagram. Decision parameters for opening a shoulder to travel based on predicting breakdown.

If an agency is unsuccessful at predicting a specific breakdown event or lacks the realtime data and/or experience to do so, opening a shoulder shortly after the onset of breakdown is usually sufficient to dissipate congestion on a freeway in several minutes. The added capacity of PTSU is substantial and nearly equal to the capacity of an additional general-purpose lane in some cases. Figure 9 shows the steps to be followed to open the shoulder shortly after the onset of breakdown.

Flowchart starts with Uncongested Conditions, moves to Observe Breakdown, moves to Open Shoulder, and ends in Dissipate Congestion. Observe Breakdown branches out to historical data, predictive models, best practices, and facility-specific needs. Open Shoulder has an astrisk.
* Assuming there are no issues with maintenance , law enforcement, environmental conditions. etc.
Source: FHWA

Figure 9. Diagram. Decision parameters for opening a shoulder to travel based on an observed breakdown.

Chapter 4 of this report presents methods an agency could use to establish their own decision parameters. As part of the preparation of this report, each method was used with data and models from typical urban freeways, and these results effectively provide initial decision parameters an agency could use as a starting point before developing their own.

Developing the Decision Support Framework

The decision of when to open and close the shoulder to traffic in realtime necessarily depends on more than just volume and speed decision parameters. The agency operator should also consider the needs of the other stakeholders (maintenance, emergency responders, transit, and local agencies). The ConOps should therefore provide the agency operator with a decision support framework to determine when to open and close the shoulder to traffic. A decision support framework (DSF) for dynamic part-time shoulder use consists of decision trees designed to answer two questions for a facility operator in realtime:

  • Should I begin the process of opening the shoulder to traffic now?
  • Should I begin the process of closing the shoulder to traffic now?

Opening a shoulder is not an instantaneous process, as shown in figure 10. It requires collection of realtime data, data transmission to the Transportation Management Center (TMC) and processing, a decision by a TMC operator to initiate opening activities, sweep activities to inspect the shoulder (with cameras or field personnel), and a second decision after the sweep to change sign displays to indicate to drivers that the shoulder is open. Interviews with U.S. and foreign agencies with PTSU facilities indicated that the sweep time generally takes 15 to 20 minutes. In other words, an operator needs to decide that they want to open a shoulder 15 to 20 minutes before it actually opens.

From left to right: Data, Collection of Real-Time Data; Transmission, Data Transmission to the Traffic Management Center (TMC) Operator; Decision, Decision by a TMC Operator to Initiate Opening Activities; Inspection, Sweep Activities to Inspect the Shoulder (Camera or Personnel); Decision, Second Decision to Change Sign Displays to Indicate to Drivers the Shoulder is Open.
Source: FHWA
Figure 10. Diagram. Events preceding the opening of a dynamic shoulder.

A DSF for D-PTSU focuses on one tactical measure: congestion. However, the basic DSF could be readily expanded to include more strategic measures of agency objectives, such as safety, mobile source emissions, noise, and transit.

The DSF presented in this report is split into two decision trees: recommending when to open the shoulder to traffic and recommending when to close the shoulder to traffic. Each one has some parameters (such as minimum time to keep a shoulder open) that can be pre-determined by an agency.

Documenting the Assumptions Used in Developing the Decision Support Framework

The DSF should state its assumptions. These may be variations on any or all of the following:

  • Dynamic lane controls are in place so that the agency can remotely open and close the shoulder within a few minutes notice.
  • The decisions to open and close the shoulder ultimately must be made by a human being before being implemented.
  • Video or loop detectors are in place and 100 percent operational to monitor lane-by-lane occupancy and speed throughout the facility continuously, including on the shoulder.
  • The DSF can be refined in later versions to include logic checks to account for detector breakdowns, discrepancies between upstream and downstream detectors, and detection errors.
  • Video surveillance or field personnel are available to verify that, before opening a shoulder, it is clear.
  • Communication links are instantly available to contact maintenance personnel, emergency responders, and freeway service patrol to obtain go/no-go recommendations from them before opening a shoulder lane.
  • Maintenance personnel, emergency responders, and freeway service patrol (FSP) managers have a means of knowing the status of the shoulder (open or closed) in realtime, their personnel geo-locations, and the conditions on the freeway.

Note that these example assumptions are effectively the prerequisites for Level 3 (i.e., D-PTSU with core hours supplemented by the capability of providing unscheduled shoulder opening) and Level 4 (i.e., fully dynamic PTSU) operations. Level 1 (i.e., S-PTSU) or Level 2 (i.e., D-PTSU with core hours and scheduled variations) could be implemented without some of these assumptions because the opening time of the shoulder would be pre-determined.

An Example Lane Opening Decision Tree

The agency's ConOps for D-PTSU should include a decision tree that its operators can follow when determining whether or not to open the shoulder to traffic. Figure 11 provides an example of a conceptual decision tree that an agency might use for opening the shoulder to traffic. This particular example presumes that the agency will not open the shoulder unless congestion is present or imminent. It presumes that the agency will not open the shoulder in any case until it has ascertained that the shoulder is free of obstructions and the agency has obtained the approval of maintenance, the FSP, and law enforcement/emergency responders (ER). These presumptions should be explicitly stated in the ConOps. Agencies may require approval from other specific stakeholders as well. If so, these additional considerations should be included in the ConOps decision tree.

Flowchart Starts. Is shoulder open? Yes, Go to Figure 12. Close Shoulder. No, Is Breakdown Imminent?* No, Keep Shoulder Closed, Check Again Soon, retry Is Breakdown Imminent?* If yes, Has the Minimum Closure Time Elapsed? If yes, Is Shoulder Clear? If yes, Get Go/No Go Maint. FSP ER **. Go Open Shoulder, and end at Go to Figure 12. Close Shoulder. * Can use volumes, speeds, or occupancy. Should also test if breakdown has already occured. ** Check with Maintenance, Freeway Service Patrol, Emergency Responders.
Source: FHWA
Figure 11. Diagram. Example shoulder opening decision tree.

This diagram shows the events for which an agency should determine a specific duration when preparing its ConOps for the D-PTSU:

  • The minimum closure time to keep the shoulder closed, once already closed.
  • The minimum time between re-checks of shoulder status.

The first decision diamond box (Is Shoulder Open?) that should be included in the ConOps decision tree verifies that the question of opening the shoulder makes sense. If the shoulder is already open, there is no need to go through the rest of the decision tree.

The second decision diamond box (Is Breakdown Imminent?) is the most critical and complex question. There are several options for implementing this decision box that the agency should consider when preparing its ConOps for the D-PTSU:

  • The agency could choose to proactively anticipate congestion and proactively open the shoulder, like the process shown in figure 11.
  • The agency could wait until congestion is present, then open the shoulder in response. This could be done if the agency's predictive capabilities are limited or if congestion unexpectedly occurs. The amount of additional capacity added with PTSU is usually sufficient to dissipate congestion within a few minutes of the opening of the shoulder.
  • In addition to congestion, the agency may also have air quality, noise, or other multimodal objectives for D-PTSU. In these cases, using off-line modeling would identify how those objectives are related to field measurements of speed and volume and identify how to set the appropriate thresholds for opening the shoulder lane.

The decision parameter values will vary between localities and between facilities. Chapter 4 provides different methods of choosing decision parameters and presents results of decision parameters-related research conducted for this project.

The third decision diamond ("Has the Minimum Closure Time Elapsed?") is to prevent frequent opening and closing of the shoulder, which could be confusing to drivers and other stakeholders (e.g., police, emergency responders). Recently closed shoulders should not be reopened too soon. In practice, D-PTSU agencies generally open the shoulder during the morning peak, the afternoon peak, or both, depending on the peaking characteristics of the facility. However, if volume fluctuates throughout the day at near-capacity levels—as might be the case on a weekend or on a rural freeway—an agency would likely want to establish a minimum time for each operating state (open/closed), such as 30 or 60 minutes.

The fourth decision diamond ("Is the Shoulder Clear?") is a safety check to make sure the shoulder is clear of obstructions (e.g., breakdowns, maintenance operations) before it is opened to traffic.

The fifth and final decision diamond ("Get Go/No Go Maint, FSP, ER") requires the operator to verify with its D-PTSU stakeholders (maintenance, FSP, law enforcement, and emergency responders) that their personnel and equipment are not on the shoulder before it is opened to traffic.

After making the decision to "Keep Shoulder Closed," figure 11 shows the next step as "Check Again Soon." This is likely a short duration of time—such as 1 to 5 minutes—because the effort for a TMC operator to check traffic conditions is relatively low. This is particularly true if software in the TMC is automatically providing a suggestion to the operator ("Keep Shoulder Closed" or "Begin Opening Shoulder") as opposed to the operator manually reviewing speed and/or volume data.

Note that the example lane opening decision tree does not address the option of partial lane openings for long facilities. It also does not address the issues of conflicting sensor results on different segments of the facility. For example, the lane opening volume decision parameter may be met in one section but not in any of the other sections. The decision tree included in the ConOps should address these additional possibilities, when they are applicable.

Example Lane Closing Decision Tree

Figure 12 provides an example of a lane closing decision tree. This particular example presumes that the agency will close the shoulder lane when one or more or of the following conditions are met:

  • The instant a blockage or incident is detected in the shoulder lane.
  • Upon request of maintenance personnel, freeway service patrol, or emergency responders.
  • If the lane has been open a minimum amount of time set by the agency and calculations or typical performance show that the lane can be closed without causing congestion.

The ConOps should document the presumptions upon which the lane closing decision tree is based.

In the case of an incident or maintenance need, it would be possible on a sufficiently long facility to close only a portion of the shoulder and not the entire length of it. The congestion threshold can be expanded to incorporate environmental and other concerns in addition to congestion. The ConOps should address these additional concerns as well as resolve conflicting detector data (such as when one section meets the volume threshold, but the others do not) and additional options for partial lane closures for long facilities.

Flowchart Startsfrom Figure 11 (Shoulder Open). Is Incident Blocking Shoulder? If Yes, Close Shoulder, Go to Open Shoulder Decision Tree. If no, Is there a Valid Request to Close?* If Yes, Close Shoulder and Go to Open Shoulder Decision Tree. If no, Has the minimum Open Time Elapsed? If no, Check Again Soon, if Yes, WOuld closing shoulder cause breakdown? If yes, Check again soon. If no, Close shoulder and go to open shoulder decision tree. * Is there a valid request from maintenance, FSP, or emergency responders to close shoulder?
Source: FHWA
Figure 12. Diagram. Example shoulder closing decision tree.

This diagram shows two suggested parameters which should be user-agency adjustable:

  • The minimum closure time to keep the shoulder open once it is already open.
  • The minimum time between re-checks of shoulder status.

The first decision diamond ("Is Incident Blocking Shoulder?") provides the decision parameters for immediately closing the shoulder should an incident occur that would block it.

The second decision diamond ("Is there a Valid Request to Close?") provides for requests coming from maintenance, law enforcement, emergency responders, or the FSP for closure of the shoulder to traffic. The request is evaluated against the criteria mutually agreed to by the various departments to determine if the shoulder should be closed. There may be a delay in closing the shoulder if the request is not urgent and the shoulder has not been open a long time.

The third decision diamond ("Has the Minimum Open Time Elapsed?") prevents frequent openings and closings of the shoulder. The agency policy sets the minimum number of minutes.

The fourth decision diamond ("Would Closing Shoulder Cause Breakdown?") is the most common reason for determining whether closing the shoulder is appropriate. The operator's experience with the facility or a predictive model is needed to determine the effect on the operations of the remaining lanes when closing the shoulder. Typically, operators have an understanding of daily traffic patterns on a freeway and when decreased volume will be sustained long enough to justify closing the shoulder.

Considerations for Permanent Shoulder Conversion

The D-PTSU ConOps may also consider addressing the potential future scenario of shoulder openings being so frequent that full-time shoulder use may be an option. The FHWA report on the Use of Narrow Lanes and Narrow Shoulders on Freeways (Neudorff, Jenior, Dowling, & Nevers, 2016) describes a performance-based analytical framework to help agencies identify if and when it may be appropriate to go to full-time shoulder use (in effect, an additional lane and narrow shoulders, which may or may not be accompanied by narrow lanes).

Much of the information contained in the primer is presented in the broader context of both performance-based planning and programming and performance-based practical design. The primer contents include case studies on the use of narrow shoulders and lanes, issues and approaches for analyzing the operational and safety impacts of narrow lanes and narrow shoulders, and the role of TSMO in supporting narrow lane and narrow shoulder operations.

In essence, the decision point for going from part-time shoulder use to full-time shoulder use occurs when the life-cycle benefit-cost ratio of full-time shoulder use exceeds that of part-time shoulder use. The benefit-cost analysis takes into account both safety effects and traffic operations effects, as described in the FHWA report. The safety effects (changes in crash frequencies, types, and severities) of PTSU are not fully researched and understood at this time, but in general there is likely a negative safety effect of opening the shoulder in a low volume period where it does provide a congestion reduction benefit. If a shoulder is open for many hours during a typical day and thus serves as a lane for recurring extended periods, it may be appropriate to convert the shoulder to a full-time lane.

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