Phase 2: Field Testing of Combined VSL/VSA and CRM for Freeway Traffic Control

Variable Speed Limit (VSL) control and Coordinated Ramp Metering (CRM) concepts have been studied in analysis and simulation in previous research projects, which have indicated that they have a significant potential to help reduce freeway congestion at recurrent bottleneck locations. These concepts now need to be tested in the field to determine whether the projected benefits could be achieved in practice in California. The proposed work continues the transition from the previous theoretical research toward field testing that was begun in contract 65A0399. If the later field test results are favorable, this could provide the basis for future widespread adoption of CRM and VSL control strategies to improve mobility and safety and reduce energy and emissions impacts of freeway congestion. The VSL speeds are assumed to be posted as Advisory (or VSA) to avoid enforcement complications.

A recent synthesis report describes both US and European techniques in ATM. The main findings include the following quantified benefit of some ATM techniques, primarily VSL and dynamic shoulder use:

  • An increase in average throughput for congested periods of 3 to 7 percent;
  • An increase in overall capacity of 3 to 22 percent (depending on road geometry and traffic situation);
  • A decrease in primary incidents of 3 to 30 percent;
  • A decrease in secondary incidents of 40 to 50 percent;
  • An overall harmonization of speeds during congested periods;
  • Decreased headways and more uniform driver behavior;
  • An increase in trip reliability; and
  • The ability to delay the onset of freeway flow breakdown.

Report has summarized the Freeway geometric design practices for ATM in Europe. It has also indicated functionalities and limits of some ATM measures such as shoulder use, lane use limit, VMS locations and feedback messages for VSL, etc. The suggested performance parameters used for evaluation include travel time and its reliability, safety, travel speed and congestion. It has also recommended that

  • To address safety concern, The Highways Agency in England developed a hazard index to systematically evaluate the potential driver safety risks and aid in its decision to implement strategies and design choices on managed motorways. The agency uses a risk-based approach for transitioning the shoulder from an emergency lane space to a travel lane. Its research has indicated that the risk of eliminating shoulders (at least for part-time use) is minimal.
  • It is important to bring all stakeholders (enforcement, trucking, traveling public, agency, and government leadership) in at the early stages of the planning and design process;
  • as traffic and congestion levels increase in the corridor, different approaches and management strategies should be considered to accommodate changing needs, risks, and appropriate tradeoffs;
  • Public perception: It has been generally recognized that the proposed operational scheme will be successful only if the public perceives it to be successful (despite what data may say).

CRM has been claimed as a great success in the CRM project on the Monash Freeway in Australia. The presentation summarized the shortcomings of traditional RM as follows:

  • delay the onset of traffic breakdown instead of preventing it
  • speeding up the flow recovery only shortens the peak period
  • improve throughput during peak period only 2~10%, but it is unsustainable when it is operated close to the capacity flow
  • crash reduction is not significant
  • travel times are still not reliable

Their key points to Coordinated Ramp Metering include:

  • all onramps must be metered
  • sensors can provide measurement/estimation of speed, volume and occupancy and are dense enough to detect shockwaves
  • signals are updated every 20s in response to freeway traffic conditions
  • signals are switched ON/OFF adaptive to traffic condition
  • reconfiguration capability: each signal can work independently or in self-organizing clusters to resolve complex traffic problems
  • queues (waiting times) are managed to best use all onramp storage spaces
  • maximize off-ramp flow
  • use occupancy to optimize throughput instead of volume and speed since occupancy is more stable in measurement
  • recommended to measure downstream of onramp to determine RM rate instead of upstream
  • when congestion is present, the operation policy is based on equity of access along the corridor, everybody shares the pain

Their approaches are summarized as follows:

  • principles of traditional and contemporary traffic flow theory
  • principle of RM
  • criteria for providing ramp signals
  • data needs
  • design guidelines
  • operation for ramp signals
  • control logic and algorithms
  • managing the arterial road interface

The project involved investments to

  • redesign the freeway mainline
  • redefine the role of and redesign ramps
  • use state-of the-art industrial technologies
  • use contemporary capacity optimization algorithm
  • install appropriate ITS devices and services

With all the systematic approaches as above, the performance achieved included:

  • provide up to 20~25% additional throughput
  • reduce travel time delay up to 50%
  • improve travel time reliability
  • reduce crashes up to 50%

For this project, we did not expect any changes in freeway mainline and ramp design, and even infrastructure including detector system. Instead, the intent was to maximally and optimally use current infrastructure for VSL and CRM to maximize the traffic flow to a feasible extent.

Relevant Publications

  • Lu, X. Y., S. Shladover, I. Jawad, R. Jagannathan, and T. Phillips, A novel speed-measurement based variable speed limit/advisory algorithm for a freeway corridor with multiple bottlenecks, 94th TRB Annual Conference, Washington D. C., Jan. 2015; to appear in TRR
  • X. Y. Lu and S. Shladover, and Z. Wang, Optimal coordinated freeway ramp metering with queue overwrite  simulation for field test, submitted to 94th TRB Annual Conference, Washington D. C., Jan. 2015
  • X. Y. Lu, D. Su, R. Horowitz, J. Spring and Z. Wang, Coordination of freeway ramp metering and arterial intersection traffic signal  field test, submitted to 94th TRB Annual Conference, Washington D. C., Jan. 2015
  • D. Y. Su, X. Y. Lu, R. Horowitz, Z. R. Wang, Coordinated Ramp Metering and Intersection Signal Control, International Journal of Transportation Science and Technology, Vol. 3, No. 2, Sept 2014, p179, 191
  • X. Y. Lu, T. Z. Qiu, R. Horowitz, A. Chow, and S. Shladover, METANET Model Improvement for Traffic Control, Int. J. of Transportation, Vol. 2, No. 2, August 2014, pp.65-88
  • X. Y. Lu, D. J. Chen, and S. E. Shladover, Preparations for Field Testing of Combined Variable Speed Advisory (VSA) and Coordinated Ramp Metering (CRM) for Freeway Traffic Control, Final Report, UCB-ITS-PRR-2014-1, Jan 2014, DOI: 10.13140/2.1.3612.5442
  • X. Y. Lu, D. Y. Su, and J. Spring, Coordination of Freeway Ramp Meters and Arterial Traffic Signals Field Operational Test (FOT), Final Report, UCB-ITS-PRR-2014-2, Jan 2014, DOI: 10.13140/2.1.4661.1207
  • D. Y. Su, R. Horowitz, and X. Y. Lu, Real-Time Estimation of Intersection Turning Proportions from Exit Counts, 93rd TRB Annual Meeting, Washington D. C., Jan.12-16, 2014
  • X. Y. Lu, J. Y. Lee, D. J. Chen, J. Bared, D. Dailey, and S. Shladover, Freeway Micro-simulation Calibration: Case Study Using Aimsun and VISSIM with Detailed Field Data, 93rd TRB Annual Meeting, Washington D. C., Jan.12-16, 2014
  • X. Y. Lu and S. Shladover, Review of Variable Speed Limits/Advisories Theory, Algorithms and Practice, 93rd TRB Annual Meeting, Washington D. C., Jan.12-16, 2014; Transportation Research Record, TRB, # 2423, 2014, pp. 15, 23.
  • Hadiuzzaman, M., Qiu, T., and Lu, X., Variable Speed Limit Control Design for Relieving Congestion Caused by Active Bottlenecks. J. Transp. Eng., 139(4), 358. 370. 2013
  • Lu, X. Y., P. Varaiya, R. Horowitz, D. Su and S. E. Shladover, A novel freeway traffic control with variable speed limit and coordinated ramp metering, Transportation Research Record #2229, TRB, 2011, pp55-65
  • S.E. Shladover, D. Su and X.-Y. Lu, Highway Capacity Increases from Cooperative Adaptive Cruise Control, 18th World Congress on Intelligent Transport Systems, Orlando, Florida, Oct. 2011, Paper No. 1081.
  • S. Shladover, D. Su, and X. Y. Lu, Impacts of Cooperative Adaptive Cruise Control on freeway traffic flow, CD ROM of 91st TRB Annual Meeting, Washington D. C., Jan. 22-26, 2012
  • S. Shladover, D. Su, and X. Y. Lu, Impacts of Cooperative Adaptive Cruise Control on freeway traffic flow, Transportation Research Record #2324, pp63-70, 2012
  • M. Hadiuzzaman, T. Z., Qiu, and X. Y. Lu, Variable Speed Limit control design for relieving congestion caused by active bottleneck, submitted to 91st TRB Annual Meeting, Washington D. C., Jan. 22-26, 2012
  • Lu, X. Y., Qiu, Z. J., Horowitz, R., Chow, A., and Shladover, S., METANET model improvement for traffic control, CD ROM of IEEE ITS Conference, Washington Univ., Washington D. C., Oct. 5-7, 2011
  • Lu, X. Y., P. Varaiya, R. Horowitz, D. Su and S. E. Shladover A novel freeway traffic control with variable speed limit and coordinated ramp metering, 90th TRB Annual Meeting, Washington, D. C., Jan. 23-27, 2011
  • Su, D., X. Y. Lu, P. Varaiya, R. Horowitz and S. E. Shladover, Variable Speed Limit and Ramp Metering Design for Congestion Caused by Weaving, 90th TRB Annual Meeting, Washington, D. C., Jan. 23-27, 2011
  • X. Y. Lu, P. Varaiya, R. Horowitz, D. y. Su, and S. E. Shladover, New Approach for Combined Freeway Variable Speed Limits and Coordinated Ramp Metering, IEEE ITS Conf., Sept. 19-22, 2010, Madeira Island, Portugal
  • X. Y. Lu, T. Z. Qiu, P. Varaiya, R. Horowitz, and S. Shladover, Combining Variable Speed Limits with Ramp Metering for freeway traffic control, CD ROM of 2010 American Control Conference, Baltimore, Maryland, June 30 - July 2, 2010