Decision Support Framework and Parameters for Dynamic Part-Time Shoulder Use:
Considerations for Opening Freeway Shoulders for Travel as a Traffic Management Strategy
Part-time shoulder use (PTSU) is a transportation systems management and operations strategy that allows use of the left or right shoulder as a travel lane during some, but not all, hours of the day. PTSU enables agencies to achieve a better balance of available supply (roadway capacity) and demand. It can generally be implemented more rapidly, at lower cost, and within a smaller footprint than a conventional widening project. In 2016, the Federal Highway Administration (FHWA) published Use of Freeway Shoulders for Travel — Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy. The 2016 guide comprehensively addresses PTSU from planning and design through implementation and day-to-day operations. It includes information on dynamic PTSU (D-PTSU), static PTSU (S-PTSU), and bus-on-shoulder (BOS).
The 2016 FHWA Publication “Use of Freeway Shoulders for Travel” comprehensively addresses PTSU.
PTSU can be implemented more quickly and cost effectively than a conventional roadway widening. It requires a shoulder that is wide enough to accommodate vehicles and that has pavement strong enough to support repeated vehicle use. D-PTSU is an active transportation and demand management (ATDM) strategy because it makes realtime adjustments to a freeway's capacity based on the traffic demand that is present, thus improving the reliability of travel time on the freeway. D-PTSU is often implemented with other ATDM strategies such as variable speed limits, dynamic lane-use control, and ramp metering. More information on ATDM is available at: https://ops.fhwa.dot.gov/atdm/index.htm.
Many agencies in the United States already employ S-PTSU with fixed operating hours on freeways. D-PTSU opens shoulders for travel beyond fixed (or static) time periods in response to traffic conditions. This report presents a framework and several processes agencies can use to identify the appropriate level of D-PTSU for their freeway facility and to set operating parameters for their D-PTSU as part of the agency's traffic management strategy. The processes of opening and closing D-PTSU are primarily ad hoc, with some agencies using predictive algorithms or control systems with volume and speed thresholds to supplement the operator's discretion in the transportation management center (TMC). Agencies may set core hours of operation to anticipate recurring congestion on the facility. Operators may also vary the opening and closing times based on observed conditions as well as the needs of maintenance, law enforcement, and emergency response personnel.
Three primary decision parameter types—fixed time-of-day, speed-based, and volume-based—are most applicable for responding to breakdowns in free-flow traffic operations under the following conditions:
- If breakdowns are frequent and predictable (e.g., every morning between 7 a.m. and 9 a.m., but rare during other times of the day), a fixed time-of-day decision parameter may be sufficient. If there are no breakdowns outside of the peak, there are few benefits to be gained from a dynamic system, and consequently there may be limited value in investing further resources in dynamic decision parameter technology.
- Volume-based decision parameters are most reliable for realtime prediction of oncoming breakdowns. Volume increases as breakdown approaches, and this incremental change often enables an analyst to predict the breakdown soon enough to initiate a sweep and open the shoulder prior to the onset of a breakdown. Volume-based decision parameters are most straightforward to apply on freeways with frequent and reasonably predictable traffic patterns (e.g., breakdown every morning peak), where the rate of volume increase, the driver population, the heavy vehicle percentage, and other parameters of the traffic stream are similar day to day.
- Speed-based decision parameters are less reliable indicators of oncoming breakdowns. In general, speed does not substantially decrease until just prior to the onset of breakdown, and there may be insufficient time to conduct a sweep prior to the onset of breakdown if a speed-based decision parameter is used. However, volume-based decision parameters should be supplemented by speed-based decision parameters. If a volume-based decision parameter fails to detect the onset of breakdown but speeds begin to decrease, then it may still be appropriate to begin a sweep and open the shoulder. The added capacity through D-PTSU often relieves the congestion quickly and enables the freeway to “recover” from short-term breakdown.
This report presents five data-driven methods for selecting and optimizing the opening time of a PTSU facility. While they are primarily presented in the context of making day-to-day decisions for a D-PTSU facility, they could also be used during concept of operations (ConOps) development to establish either hours of operation for S-PTSU or core hours of operation for D-PTSU. The five methods are:
- Demand-to-Capacity Patterns – Using sensors or traffic counts on the facility, an operating agency assesses historical demand profiles to determine levels of congestion relative to the available facility base capacity as well as the expected capacity with shoulder use.
- Empirical Performance Data – Using whole-year travel time reliability data, an operating agency explores the frequency and pattern of breakdown events to identify times when the facility experiences congestion.
- Macroscopic Decision Parameter Optimization – Using the Highway Capacity Manual (HCM) freeway facilities method, an operating agency examines different types of decision parameters (speed vs. volume-based) and decision parameter values for a facility.
- Microscopic Decision Parameter Refinement – Using calibrated microsimulation tools, an operating agency simulates the facility in question with the initial proposed decision parameter algorithm.
- Monitoring and Adjustment – The operating agency uses realtime operating experience to adjust the decision parameter values and open or close the shoulder at different thresholds.
Other research documented in this report includes a synthesis of D-PTSU literature and a summary of known D-PTSU facilities worldwide. D-PTSU implementations to date have been located on freeway facilities, thus this report focuses on freeway applications of D-PTSU.