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Adaptive Signal Control

Adaptive Signal Control Technologies (ASCT)

What are Adaptive Signal Control Technologies?

The variability and unpredictability of traffic demand on arterial systems often outpace the ability of local and State agencies to update signal timings so that signalized intersections operate efficiently and do not cause congestion and delays to motorists and pedestrians. The 2007 National Traffic Signal Report Card rated the Nation's traffic signal management and operations practices with a letter grade of "D" and estimated that poor traffic signal timing contributes to as much traffic congestion and more than 295 million vehicle-hours of delay on major roadways alone. Conventional signal systems do not use pre-programmed, daily signal timing schedules that do not monitor system performance, nor can they adjust automatically to accommodate traffic patterns that are different from the peak periods during which they were designed to operate. Adaptive signal control technologies adjust when green lights start and end to accommodate current traffic patterns to promote smooth flow and ease traffic congestion. The main benefits of adaptive signal control technology over conventional signal systems are that it can:

  • Automatically adapt to unexpected changes in traffic conditions.
  • Improve travel time reliability.
  • Reduce congestion and fuel consumption.
  • Prolong the effectiveness of traffic signal timing.
  • Reduce the complaints that agencies receive in response to outdated signal timing.
  • Make traffic signal operations proactive by monitoring and responding to gaps in performance.

How Does It Work?

By receiving and processing data from sensors to optimize and update signal timing settings, adaptive signal control technologies can determine when and how long lights should be green. Adaptive signal control technologies help improve the quality of service that travelers experience on our local roads and highways.
The process is simple. First, traffic sensors collect data. Next, traffic data is evaluated and signal timing improvements are developed. Finally, the adaptive signal control technology implements signal timing updates. The process is repeated every few minutes to keep traffic flowing smoothly. Traditional signal retiming might only repeat this process every 3 to 5 years.

The traditional signal timing process is time-consuming and requires substantial amounts of manually collected traffic data. Traditional time-of-day signal timing plans do not accommodate variable and unpredictable traffic demands. This results in customer complaints, frustrated drivers, excess fuel consumption, increased delays, and degraded safety. Customer complaints is the most frequently cited performance measure in operations surveys conducted by the FHWA. In the absence of complaints, months or years might pass before inefficient traffic signal timing settings are updated. With adaptive signal control technologies, information is persistently collected and signal timing is updated continually.

A wide variety of adaptive signal control technologies are available to  operate in varied environments. A wide variety of adaptive signal control technologies are available to operate in varied environments.

Why Adaptive Signal Control Technologies?

  • Adaptive signal control technologies deliver improved service to road users.
    • Outdated traffic signal timing currently accounts for more than 10 percent of all traffic delays. On average, adaptive signal control technologies improve travel time by more than 10 percent. In areas with particularly outdated signal timing, improvements can be 50 percent or more.
    • Adaptive signal control technologies also react to unexpected events, such as crashes and special events. By adjusting traffic signal timing in real-time to reflect actual conditions on the road, travelers enjoy a more reliable trip.
    • Studies indicate that crashes could be reduced by up to 15 percent through improved signal timing. Adaptive signal control technology can reduce the intersection congestion that causes many crashes.
  • Adaptive signal control technologies solve problems for signal operators.
    • Performance management and acquiring the information necessary to measure performance are challenges facing many transportation agencies. Adaptive signal control technologies capture a rich set of data that signal operators can use to monitor their performance.
    • By solving traffic problems as they occur, adaptive signal control can reduce citizen complaints and frustration. Adaptive signal control technology is a proactive step that signal operators can take to improve service.
  • Adaptive signal control technologies provide value.
    • The costs of congestion and delay to road users are substantial, and adaptive signal control technology delivers benefits to users that far outweigh its cost.
    • Adaptive signal control technologies also provide value directly to signal operators. By extending the effectiveness of traffic signal timing plans, implementing adaptive signal control technologies can yield direct savings by reducing the frequency of manually retiming signals.

Alternative Adaptive Traffic Signal Control Technologies

In the United States, several adaptive systems are available from multiple vendors. Agencies should evaluate their needs, system requirements and operations, and maintenance capabilities through a system engineering process to evaluate if and which adaptive signal control technology(s) best meet their agency needs and requirements. Each system has specific requirements, and each will produce improved levels of performance that are consistent with agency commitment to management and operations programs.

A recent synthesis report, NCHRP 403, describes readily available foreign and domestic adaptive controls systems; of note are several systems currently deployed in the United States. The Split cycle Offset Optimization Technique (SCOOT) is the most widely deployed adaptive system in existence. It was develop in the United Kingdom. ACS-Lite was developed as part of an ongoing Federal Highway Administration Research Program at the Turner Fairbank Highway Research Center in the United States, to improve progression and phase utilization for small scale arterial systems of 30 or fewer traffic signals, producing smoother flow and fewer traffic delays. The Sydney Coordinated Adaptive Traffic System (SCATS) was developed in Australia, and matches traffic patterns to a library of signal timing plans and scales split plans over a range of cycle times. Another effective system is the Real Time Hierarchical Optimized Distributed Effective System (RHODES), which uses a peer-to-peer communications approach to communicate traffic volumes from one intersection to another in real time. InSync developed by Rhythm Engineering (Lenexa, Kansas) combines a strategy of global and local intersection optimization methodology to improve arterial progression while reducing side street and left turn delay. There are many others in existence and in development.

When adaptive signal control technologies are used, overall travel  times decrease. When adaptive signal control technologies are used, overall travel times decrease.
Adaptive signal control technologies can reduce stop time at intersections. Adaptive signal control technologies can reduce stop time at intersections.

How should ASCT be selected?

The key to selecting an appropriate ASCT is to identify what level of functionality is required to meet operational objectives and how that functionality can be supported within the operations and maintenance capabilities of the operating agency. Systems Engineering is a process by which stakeholders are engaged to identify customer needs and expectations in order to align these with required functionality to implement a quality product that meets current and future needs.

Where is ASCT effective?

ASCT is effective where variability and unpredictability in traffic demand results in excessive delay and stops that cannot be reasonably accommodated by updating coordinated signal timing parameters on a frequency consistent with agency traffic signal operations objectives.

How much does it ASCT Cost?

The cost of ASCT typically ranges between $6,000-$50,000 depending on the current infrastructure, communications and detection requirements of the selected system. Depending on the pricing strategy of the vendor, the cost per intersection may decrease as the number of intersections included in the project increases. Costs include:

  • Licensing (Number of intersections)
  • Warranty (What's covered, for how long)
  • Training and Support (Depth, In Person, Phone Support)
  • Hardware
    • Platform Cost — Servers, Processors
    • Communications
    • Detection — Detection cost should be evaluated to consider the specific needs of the system and how flexible the system operates with existing configurations)
  • Operations & Maintenance (Reliability, Complexity, Failure Mode)

How can my agency implement ASCT?

ASCT must be procured through a vendor and cannot be provided by the FHWA. The FHWA Resource Center can provide technical assistance to guide agencies through the process of evaluating if ASCT will meet their needs, objectives, operations and maintenance capabilities and resource constraints to make an informed implementation decision.

How to fund ASCT implementation?

A number of funding sources can support traffic signal management and operations activities, equipment and specifically ASCT. In practice, however, funding for traffic signal system management and operations must often rely on the discretionary budgets of individual jurisdictions and/or agencies. The implementation and operating costs for ASCT is eligible for Federal reimbursement from National Highway System and Surface Transportation Program funding. For projects located in air quality non-attainment and maintenance areas, and in accordance with the eligibility requirements of 23 USC 149(b), Congestion Mitigation and Air Quality Improvement Program funds may be used for operating costs for a 3 year period, so long as those systems measurably demonstrate reductions in traffic delays. Operating costs include labor costs, administrative costs, costs of utilities and rent, and other costs, including system maintenance costs, associated with the continuous operation of the system.

Federal funds are made available through a specific process that ensures that recipients comply with all applicable federal laws. It is critical that agencies work through the transportation planning process collaboratively with their Metropolitan Planning Organization (MPO), State Department of Transportation and FHWA Division office. Working through the Systems Engineering process offers the potential to align agency traffic signal management objectives and needs with regional planning goals and objectives to identify funding opportunities. The Planning for Operations Program offers key resources to navigate and incorporate management and operations activities into the planning process.

For assistance in engaging the process contact the FHWA Division Office in your state; the FHWA Resource Center Operations Technical Service Team or the FHWA Office of Operations Arterial Management Team.


Page last modified on August 12, 2013.
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