Traffic Analysis Tools Volume IX: Work Zone Modeling and Simulation
A Guide for Analysts
Wisconsin DOT Work Zone Signal Optimization
Wisconsin DOT Work Zone Signal Optimization
|Work Zone Characteristics|
|Approach||Traffic Signal Optimization|
|Type||Type I and IV|
|Work Zone Size||Small|
Use of Signal Optimization Tools in Work Zone Traffic Analysis
Signal optimization tools such as Passer, Synchro/SimTraffic, and Transyt 7F have a variety of applications for work zone analysis, especially in urban and suburban environments. Broadly speaking, these applications can be grouped in three categories:
- Preparing timing plans for temporary signals used to manage traffic within a construction site.
- Adjusting signal timing on corridors that are directly impacted by construction.
- Adjusting signal timing to improve progression on corridors that serve as alternate routes or detours around a work zone.
Temporary Signals. Figure 29 shows an example of the use of Synchro/SimTraffic to optimize the timing of a temporary traffic signal. In this case, two-way one-lane operation will be in effect during a bridge construction project (in other words, eastbound and westbound traffic will be sharing a single lane). Synchro’s Ring/Barrier Editor was used to create a configuration that mimics the operation of the temporary signal by alternately sending eastbound and westbound traffic along the restricted section. Synchro’s signal optimization algorithm was then used to establish a timing plan that minimizes traffic delays. The analysis also provides an indication of the extent of queuing on the approaches to the one-lane segment, which is useful in determining whether access to side roads will be blocked by queued traffic.
Figure 29 Synchro/SimTraffic Model of a Work Zone with Two-Way One-Lane Operation
*Note the westbound vehicles queuing while eastbound traffic is allowed to proceed.
This method can also be used to evaluate the impact of work zone length on capacity and throughput for sites with two-way one-lane operations. As shown in Figure 30, the capacity of two-way one-lane sections is sensitive to the length of the restricted section. Therefore, in many cases there is a trade-off between what is convenient for construction operations and what is acceptable in terms of traffic impact.
Figure 30 Capacity vs Length for Two-Way One-Lane Flagging Operations
Adjusting Timing on Corridors Affected by Construction. Normally, signal timing plans are developed based on the assumption that all of the lanes that exist at each intersection will be available for traffic to use. This assumption may not be true during construction. For example, take an intersection where a two of the three lanes have been closed to traffic. In this case, all traffic is directed to use the right lane, severely impacting the capacity of the signalized intersection.
In such situations, to avoid excessive queuing and delay it may be necessary to make fundamental changes in the signal timing at individual intersections or along an entire corridor. In the example shown in the photo, it may be desirable to increase the cycle length to compensate for the fact that left, thru, and right turning vehicles are sharing a single lane. To maintain good traffic progression along the corridor, signal offsets may need to be adjusted to account for reduced travel speeds. In addition, temporary changes in access to business properties along the corridor may affect turn patterns, requiring adjustments in signal phasing and splits. The use of a signal optimization tool allows all of these variables to be addressed comprehensively.
Adjusting Timing on Parallel Routes. The Daniel Webster Hoan Memorial Bridge carries Interstate 794 over the Milwaukee River in Milwaukee, Wisconsin. As shown in Figure 31, on December 13, 2000 there was a structural failure on one span of the bridge. The failure required immediate lane closures, resulting diversion of all traffic to other routes.
To accommodate increased traffic on the arterial street that runs directly parallel to I-794, the City of Milwaukee used signal optimization tools to prepare a revised traffic signal timing plan for the Kinnickinnic Avenue/First Street corridor (WIS 32). The revised signal timing plan was implemented less than 48 hours after the incident occurred (and at minimal cost). It increased the green time allocated to north-south thru traffic, and reduced the amount of time allocated to side streets. The revised signal timing is believed to have been instrumental in reducing traffic delays and minimizing the overall impacts of the bridge failure and subsequent reconstruction activities.