Decision Support Framework and Parameters for Dynamic Part-Time Shoulder Use:
Considerations for Opening Freeway Shoulders for Travel as a Traffic Management Strategy
Chapter 5. Closing the Shoulder
This chapter describes reactive and predictive methods for determining when a shoulder should be closed to traffic. It discusses traffic-related and non-traffic-related (i.e., maintenance, weather, incidents, emergency response, and safety) decision parameters.
Realtime and Predicted Traffic Conditions
To maximize the safety benefits dynamic part-time shoulder use (D-PTSU), it is desirable to close the shoulder to traffic as soon as it is no longer needed to prevent congestion. At the same time, it is NOT desirable to cause congestion on the remaining lanes of the freeway when the shoulder is closed.
The shoulder should be closed based upon predicted traffic operations after closure. In essence, a lane is being removed and the operator needs to ensure that the remaining lanes can carry the added traffic that shifts off of the shoulder.
If opening the shoulder eliminates congestion on the remaining lanes of the freeway, then a threshold based on speed alone will not be useful for determining when the shoulder can be closed without creating congestion on the remaining lanes of the freeway.
A volume threshold works best for determining when a shoulder can be closed without congesting the remaining lanes. As an initial threshold, the agency might set its volume target such that when the shoulder is closed, the resulting volumes per lane in the general-purpose lanes do not exceed the agency's target for opening the shoulder. As long as volumes are consistently decreasing at this time, this will eliminate unnecessary feedback in the decision support framework and avoid starting a process to re-open the shoulder as soon as it is closed.
As with shoulder opening thresholds, any computed shoulder closure threshold should be adjusted as the agency operator gains experience with the operations of their specific facility. Freeway operations should be monitored closely until the operator is comfortable with the shoulder opening and closing thresholds, and then monitored regularly but less frequently for changes in conditions that may warrant further adjustment of the thresholds.
Maintenance, Incidents, and Emergency Response
Incidents, weather, and the maintenance and emergency responses needed to address those conditions may all require closure of the shoulder before volumes decrease to a level where closure would normally occur. Agency policies and agreements with emergency responders will more precisely determine how and under what conditions the shoulder will be closed to traffic for reasons other than sufficiently low traffic volumes. In general, shoulders are closed as quickly as possible in these situations; this ability is an advantage of D-PTSU over static part-time shoulder use.
Safety Considerations
Part-time shoulder use is founded upon the belief that shoulders are an inherently good and useful feature of a freeway to retain. They provide both a margin of error for vehicles to depart a travel lane without departing the roadway as well as a refuge for disabled vehicles. When shoulders are opened for travel, it is during congested periods, and the temporary loss of the shoulder is presumably offset by the reduction in congestion-related crashes. Before-after studies of PTSU implementation have found varying results with regard to full-day safety. National Cooperative Highway Research Program Project 17-89 Safety Performance of Part-time Shoulder Use on Freeways, is currently underway and scheduled to be completed in 2020. It is anticipated that the results of this research will further inform this discussion.
