From 1999 to 2001, the City of Los Angeles Department of Transportation developed and deployed an Adaptive Traffic Control System (ATCS) that automatically adjusts traffic signal timing at 375 intersections. The ATCS collects and analyzes extensive detector data to determine real-time traffic demand and control cycle length, phase split, and offset on a cycle-by-cycle basis. The most appropriate signal timing for existing conditions is implemented within one signal cycle. For each intersection, the ATCS provides Adaptive, Critical Intersection Control, and Critical Link Control functions. In Adaptive mode, the cycle length changes based on prevailing traffic conditions. The Critical Intersection Control function adjusts splits of green time for each phase based on the demand of each approach. The Critical Link Control function alters offsets to minimize stops on the approaches with the highest volumes.

The City evaluated the ATCS by comparing its operation to the Urban Traffic Control System (UTCS) that had been in operation since 1984. Intersections in the Mar Vista and Brentwood areas of the city were selected for the evaluation. "Before" and "After" data were collected at the selected intersections over four days during daylight hours under clear, dry weather conditions. The evaluation period covered the morning (7:30 to 9:30 AM), midday (11:30 AM to 1:30 PM), and evening (4:00 to 6:00 PM) peak periods during typical weekdays (Tuesday, Wednesday, and Thursday). The "Before" condition was defined as the system operating in time-of-day mode under UTCS with a basic Critical Intersection Control function. The "After" condition was operation under the full ATCS Adaptive function.

The floating car method and a laptop computer were used to measure the travel time, stops, and delays. Data on stops, delays, and traffic volumes were also collected using a feature of the ATCS that generated traffic data reports using system detector data gathered every second. At one intersection, a Critical Movement Analysis tool was used to collect detector data for evaluation of capacity improvements.

Evaluation results indicated that the ATCS reduced travel time by 12.7 percent, decreased average stops by 31.0 percent, and lowered average delay by 21.4 percent. Travel time and stops were reduced for all time periods and delay declined for AM and PM peak periods. The results of the capacity analysis conducted at a single intersection showed that the ATCS has the potential to increase throughput or operational capacity. Additional data must be collected and analyzed before definitive conclusions on capacity improvements can be made.