Operation of Traffic Signal Systems in Oversaturated Conditions, Volume 2 – Final Report (2014) / Chapter Skim
Currently Skimming:
Pages 305-346
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 305... ...
Appendix A: Literature Review Summary of the Literature Review Development of strategies to handle oversaturated conditions is not a new topic of consideration. The research team found a wide variety of work in both diagnosis and estimation of oversaturation and control strategies and scenarios.
|
From page 306... ...
Literature Review on Diagnosis of Oversaturated Conditions Although there has not been a significant amount of literature devoted to how to manage oversaturated traffic signal systems, there has been even less effort devoted to the identification of oversaturated conditions, with both spatial and temporal extent. Many management strategies as detailed in Part 2 of the literature review assume that arrival volumes can be somehow known and the oversaturated conditions can be accurately predicted based on this information.
|
From page 307... ...
Eq. A-3 Direct application of the above models, however, is difficult.
|
From page 308... ...
ii) Oversaturated Operations: is characterized as a situation where a queue exists and it has grown to the point where upstream traffic operations are adversely affected.
|
From page 309... ...
1988) investigated the queue-based intersection delay under oversaturated conditions.
|
From page 310... ...
Figure A-1. Saturation and residual queues (Source: Denney et al., 2008)
|
From page 311... ...
becomes over time, a more and more significant component of over-all intersection delay. How this delay accumulates is extremely difficult to measure.
|
From page 312... ...
Figure A-3. Random and overflow delay models compared (ource: Hurdle, Roess, et al., 2004)
|
From page 313... ...
volume-occupancy diagram. Although most of the data in these studies are generated from simulations, these general ideas will have a contribution on our research on the identification of oversaturation.
|
From page 314... ...
Eq. A-4 where, is the proportion of vehicle arriving on green; is the number of vehicles arriving on green; and is the number of vehicles arriving on red.
|
From page 315... ...
It is necessary to point out that the high green utilization of a given phase is not always necessarily due to oversaturation. In some cases, good coordination design can generate high utilization on a phase by synchronizing the arrival of the incoming platoon with the start of green.
|
From page 316... ...
Figure A-7. Queue estimation in SCOOT (Source: Martin, 2006)
|
From page 317... ...
that the cycle time of this intersection is gradually increased (and decreased at times) in conjunction with the measurement(s)
|
From page 318... ...
OPAC or algorithm enhancements that allow OPAC to estimate oversaturation or adjust its flow profile estimates during congested conditions. Figure A-9.
|
From page 319... ...
by time of day. As such, during rush hours the estimated queues can be made to dissipate more slowly on oversaturated approaches allowing the baseline adaptive algorithms to operate as if the intersection were still under-saturated on all approaches.
|
From page 320... ...
Figure A-10. Phase utilization measures from the ACS-Lite adaptive control system (Source: Gettman, 2005)
|
From page 321... ...
Queue Estimation in the German ACS/BALANCE/MOTION Adaptive Control Systems ACS, BALANCE, and MOTION are all adaptive control systems developed in Germany that are based on the estimation of queues to recognize the traffic state (Mueck, 2005)
|
From page 322... ...
Figure A-12. Maximum back-up length (veh)
|
From page 323... ...
could be problematic when the arrival rate of traffic flow fluctuates greatly, which is not unusual with the gating effect of an upstream traffic signal. Link Load in the TUC Adaptive Control System The TUC (Traffic-responsive Urban Control)
|
From page 324... ...
The accuracy of both techniques highly depends on the input flow measured by the upstream sensor and thus the location of such a sensor is crucial. Generally, this sensor should be located sufficiently far back in order to avoid frequent queue spillback, but at the same time, the sensor should not be located so distant that "driveway activity between the sensor and the stop line seriously degrades the accuracy of the predicted arrival flow profile" (Bonneson, 2005)
|
From page 325... ...
Figure A-15. Process (a)
|
From page 326... ...
converted into segment-based travel time. An estimate of delay, and perhaps the length of standing queues could then be made by comparing the running time with the free-flow running time.
|
From page 327... ...
Control Strategies for Oversaturated Conditions Control strategies for managing traffic congestion in oversaturated conditions seek to improve facility capacity or reduce facility demand. Enhancements to the roadway capacity can be achieved by increasing the physical capacity of the roadway system or by maximizing the operational capacity by improving the traffic signal controls.
|
From page 328... ...
(b) System: Phases adjusted for progression, equity offsets, splits allocated to available queue storage.
|
From page 329... ...
approaches' green durations to change from minimum to maximum and vice versa (Gazis et al., 1968)
|
From page 330... ...
isolated intersection and then extended the analysis to a network of multiple intersections. Longley suggested that the stability of the network may be improved by the use of coordinated signals, and by establishing stability criteria for the parameters of each controller.
|
From page 331... ...
Offset Design Schemes for One-Way Progression Pignataro et al.
|
From page 332... ...
Recent research by Baird, et al (2007) and Smaglik, et al (2007)
|
From page 333... ...
their operation, possibly at the expense of the arterial street network (Herrick and Messer, 1992)
|
From page 334... ...
actual traffic demand may render the signal timing plans established by this optimization approach ineffective. Real-Time Adaptive Control Algorithms Most of the techniques reviewed so far were originally designed to be established "off-line" and operated when oversaturated conditions were anticipated during a pre-set time.
|
From page 335... ...
addition, each cycle in SCOOT the split optimizer can increase or decrease split times by a small amount (+/- 4 seconds) to keep all of the splits below 90% saturation (which could be a user-defined value, but not typically modified)
|
From page 336... ...
offsets in this situation but rather increase the green time of the phase that the queue is served by. The combination of biasing, congestion offsets, fixed offsets, and the normal tuning operation allows significant flexibility in the operation of SCOOT for offsets, although these features must be used carefully together.
|
From page 337... ...
available green time for the stretch phase. This approach uses existing engineering knowledge of field conditions to allow biasing of the additional split time to the phases that are likely to become oversaturated and cause more damaging results than oversaturation at other stages.
|
From page 338... ...
long queue formations. Quinn (1992)
|
From page 339... ...
compare four signal timing tools, including RT/IMPOST, PASSER, TRANSYT, and SYNCHRO. The result showed that RT/IMPOST policy yielded improved network travel speed and delay during oversaturated conditions.
|
From page 340... ...
conditions. Even fewer control strategies have been developed specifically to speed recovery from congested conditions.
|
From page 341... ...
3. Maximize throughput by avoiding the provision of green time that cannot be used or is inefficiently used because traffic cannot flow during the green periods.
|
From page 342... ...
Since a reduced cycle time decreases the capacity of the intersection as a whole during free flow conditions, it must be allowed to revert to its original value as soon as it has achieved its objective to manage the oversaturated condition (Quinn, 1992)
|
From page 343... ...
Abbas et al.
|
From page 344... ...
determine when a phase is oversaturated and has an estimate of how much, but does not directly model queue length. SCOOT more accurately approximates the queue on a link by modeling the dynamics of flow on the link.
|
From page 345... ...
• The if…then queue management concepts in SCOOT and SCATS. These approaches for initiating gating operation and triggering jumps or reduction in cycle times are directly relevant to this research project.
|
From page 346... ...
implemented by practitioners when the condition is intermittent. Undue delays might be induced when the corresponding improvement to throughput is not significantly realized.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.