From Tech Transfer Newsletter, Fall 2010 » printer-friendly

Every Day Counts
Adaptive Signal Control Technology

Outdated fixed interval signal timing contributes to traffic congestion. Adaptive signal control technologies can use real-time traffic information to reduce congestion by determining which lights should be red and which should be green.

Improving Traffic Flow

Wait, go, stop, wait, wait some more; most drivers have spent time fuming at red lights. Maybe the intersection was empty, yet the light stayed red for a maddening amount of time. Or perhaps the road is so congested that you have to wait three or more full light cycles before you can make a left turn. Why don't traffic lights adjust to actual conditions?

Adaptive Signal Control Technology (ASCT), in conjunction with well-engineered signal timing, can do just that. ASCT refers to technologies that capture current traffic demand data to adjust traffic signal timing to optimize traffic flow in coordinated traffic signal systems. By receiving and processing data from strategically placed sensors, ASCT can determine which lights should be red and which should be green. ASCT helps improve the quality of service that travelers experience on 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, ASCT implements signal timing updates. The process is repeated every few minutes to keep traffic flowing smoothly. On average, ASCT improves travel time by more than 10%. In areas with particularly outdated signal timing, improvements can be 50% or more.

Faster Responses to Traffic Conditions

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 produces customer complaints, frustrates drivers, and degrades safety. In the absence of complaints, months or years might pass before inefficient traffic signal timing settings are updated. With ASCT, information is collected and signal timing is updated continuously.

Special events, construction, or traffic incidents typically wreak havoc on traffic conditions. While large-scale construction projects and regular events can be anticipated, determining their impact on traffic conditions can be extremely difficult. Time-of-day signal timing cannot accommodate other disruptions, such as crashes.

ASCT Tools

Full-scale ASCT technologies are appropriate for large-scale systems of 100 or more traffic signals and on grid systems. These technologies are fully integrated into central systems that operate on a second-by-second basis and have substantial communications and detection systems overhead. They require a high level of operations and maintenance capability and often entail a traffic management center with 24-hour staffing.

A variety of choices are available from many vendors, with more in development. Available adaptive signal control technologies include the Split Cycle Offset Optimization Technique (SCOOT), Sydney Coordinated Adaptive Traffic System (SCATS), Real Time Hierarchical Optimized Distributed Effective System (RHODES), and Optimized Policies for Adaptive Control (OPAC) “Virtual Fixed Cycle.”

Use of ASCT in California

Transportation agencies in California are actively using ASCT tools, including SCOOT and SCATS. According to FHWA, Santa Barbara and Anaheim use SCOOT; Santa Rosa, Menlo Park, Sunnyvale, and Chula Vista use SCATS; Los Angeles uses its own custom system; and several additional cities use other technologies.

The City of Los Angeles uses an Automated Traffic Surveillance and Control (ATSAC) system that was developed in-house to meet the specific needs of the city. The system includes more than 17,000 detectors and is in use on 3,200 of the city’s 4,400 traffic signals. The system was initiated to accommodate the increased traffic demands of the 1984 Olympics and has grown ever since. A study conducted in 2001 analyzed the effectiveness of the system, finding it improved travel time by 13%, decreased stops by 31%, and reduced delay by 21%. For more information about ATSAC, visit: and

ACS Lite

Because full-scale ASCT is expensive, FHWA has developed a cost-effective alternative: ACS Lite. ACS Lite is a specific ASCT—developed by FHWA through a public-private partnership—that shatters many of the barriers to wide deployment of ASCT by reducing the cost, complexity, management, and operations burden typically associated with adaptive control. Significantly, ACS Lite can be used to retrofit existing traffic signals using conventional control equipment, communications, and traffic sensors. Also, ACS Lite does not require a centralized management system.

ACS Lite provides adaptive control for small-scale systems of 30 or fewer traffic signals on linear arterials. It cannot be used to coordinate grid systems. It is signal management software that runs on a computer located either in an office or in the field. The software adapts traffic signals to current conditions every few minutes, and within agency operator constraints, by:

  • Measuring and balancing the use of green periods displayed to motorists moving through the intersection.
  • Measuring, predicting, and maximizing vehicle arrivals on green signals by adjusting the onset of main-street green time.

For use in closed-loop systems, which represent 90 percent of the traffic signal systems in the United States, ACS Lite is effective.

Use of ACS Lite in California

The City of Anaheim has initiated implementation of a complete ACS Lite adaptive traffic control system at five signalized intersections in the vicinity of Disneyland. The project will provide enhanced signal control and timing coordination within a small area consisting of five signalized intersections along the approach to the theme park.

Also, several field tests are being conducted across the country to integrate and test the ACS Lite software with traffic signal controllers manufactured by four companies. A site in El Cajon, CA was chosen as one of the test sites.


At this time, real-time management systems have been deployed on less than 1% of existing traffic signals nationwide. FHWA is now working to expand the use of these technologies in California and across the country through the development and continued evaluation of the low-cost ACS Lite technology and by providing training and technical assistance on the technologies to public agencies.

To Learn More

FHWA’s ASCT website includes detailed information about the technology, as well as resources, presentations, FAQs, and more.

Top of Page Top of Page