Decision Support Framework and Parameters for Dynamic Part-Time Shoulder Use:
|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).
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:
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:
Agencies generally use one or more of the following decision parameters to determine when they should close the shoulder to travel:
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.
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.
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.
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:
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.
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.
The DSF should state its assumptions. These may be variations on any or all of the following:
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.
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.
This diagram shows the events for which an agency should determine a specific duration when preparing its ConOps for the D-PTSU:
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 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.
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 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.
This diagram shows two suggested parameters which should be user-agency adjustable:
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.
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.
United States Department of Transportation - Federal Highway Administration