By: Farhad Pooran, Ph.D., P.E.
Traffic-signal retiming is one of the most cost-effective ways to improve traffic flow and one of the most basic strategies to help mitigate congestion.
But outdated or poor traffic-signal timing accounts for a significant portion of traffic delay in urban areas.
More than half of the signals in North America are currently in need of repair, replacement or upgrading. In the current economy, with shrinking budgets and staff, maintenance activities to traffic signals are often delayed or canceled.
The Federal Highway Administration (FHWA) has recognized the critical role that traffic-signal timing plays within the overall transportation network, because it allows for the opportunity to improve the mobility and safety of travelers and also contributes environmental benefits. However, because travel-demand patterns change over time, effective signal-timing plans should be periodically updated to maintain intersection safety and efficiency.
Today, there are millions of traffic signals in the U.S.—all sources of significant frustration for the public when not operated efficiently. Urban highways are holding more vehicles than ever before at a time when driver demographics are becoming increasingly complex (older drivers, more distractions, larger vehicles, etc.).
At the same time, traffic signals at a busy intersection in a typical urban area might direct the movement of 200,000 vehicles or more per day. In Los Angeles, for example, there are as many as 10,000 intersections that direct thousands of traffic movements per day.
Even with what many view as successful signal-timing systems, there are many that could be improved by updating equipment or by simply adjusting timing plans.
Timing to the optimum
Optimized Policies for Adaptive Control (OPAC), originally developed for the FHWA, is a sophisticated open-system platform that allows jurisdictions to adjust timing parameters of their signal systems to reduce congestion and increase mobility. OPAC is a distributed control strategy that features a dynamic algorithm that calculates signal timing to decrease the risk of intersection delays and stops. The main component of OPAC that allows for success is that it evaluates traffic-flow parameters at each intersection in real-time and adapts to traffic flow. Major shifts in traffic are recalculated every few seconds, and adjustments are made with each traffic cycle.
In addition to traffic-flow benefits, OPAC also showed significant improvements in fuel consumption, air quality, maintenance and operations cost and facilitating automated collection of traffic data. Years of operation of this technology in multiple locations has proven the benefits of the OPAC adaptive-control system in responding to various traffic conditions. As a result, the system has been running all day in order to handle day-to-day operation of certain traffic operation centers.
The adaptive-signal-control software was the key element in addressing the shortcoming of the existing signal systems in Pinellas County, while providing new traffic-management strategies along the roadway systems.
Getting real-time
One example of how OPAC transformed traffic-signal systems was in 2006 when the Florida Department of Transportation and Pinellas County officials decided to respond to the need for change in traffic patterns and travel time, as it was the most densely populated county in Florida.
In addition to the high volume of residents, Pinellas County travelers have limited access to highways and severe seasonal tourism variations, which place a heavy burden on its surface transportation network. The county officials’ objective was to use real-time technology such as OPAC to improve the roadway capacity within the county, implement video monitoring and changeable message signs in order to manage collisions and lay the communications infrastructure for future traffic-signal technologies.
For initial development phases, Pinellas County chose major corridors with high variations in traffic congestion throughout the year—corridors that could not be expanded economically.
The adaptive-signal-control software was the key element in addressing the shortcoming of the existing signal systems in Pinellas County, while providing new traffic-management strategies along the roadway systems. Before implementing the technology, several areas needed to be addressed, including organizational and funding structures that would support the technical decisions.
Pinellas County has 23 cities with three separately operated, computerized signal systems. Instead of a quick-fix option, Pinellas County chose to invest in a sustainable solution. Aggressive management of travel demand enabled Pinellas County to increase mobility and minimize harmful emissions and energy consumption.
U.S. Highway 19 in Florida has two sections currently operating under OPAC. The first OPAC section installed has been operating for almost three years and controls 16 intersections spaced at an average distance of 1 mile within the city of Clearwater. The average speed of this roadway is 50-55 mph with between 75,000 and 90,000 vehicles per day.
The second OPAC section on south U.S. 19, within the city of St. Petersburg, was installed in a subsequent Stage 2 contract controlling eight intersections spaced at an average of 0.25-0.5 mile apart with average speeds of 35-40 mph ranging between 51,000 to 67,000 vehicles per day. This section has been operating for less than one year. This 25-mile roadway segment runs the entire length of the county in a north-south direction and is the heaviest traveled roadway in the county. It serves as the primary commuter, commercial and business route in the county and travels through nine separate municipalities.
Faster, sooner, cheaper
The results of the data show travel times through the 10-mile section where OPAC was installed were reduced by 2%-25% with an average reduction of 7.5%. After implementation of the new software, the travel time dropped from an average of 19 minutes to 14.5 minutes. The most dramatic benefits were shown in the peak travel times. In the early morning and afternoon peak times, the software reduced travel times in the primary direction by an average of 25%.
The benefits from the reductions in travel time for the adaptive software represent reductions in stops, delay and fuel consumption while increasing the overall speed along the corridor. The study results for U.S. 19 indicate a $600,000 annual savings in fuel costs to the motorist, while the study results for S.R. 60 demonstrate a $750,000 annual savings in fuel costs to the motorist.
An overall evaluation of the total implementation, beginning to end, also was done and looked at the savings in time (lost labor), delay and fuel consumption. A benefit/cost ratio for the new installation turned out to be 7:1. For every dollar spent on the new system, there was a return to the motoring public of $7, far exceeding expectations.
In both cases accident data was evaluated and showed improvements in the number and severity of accidents on the main corridor. The improvements included a 50% reduction in rear-end accidents on U.S. 19, indicating a much smoother and predictable flow of traffic. As a result, Pinellas County has expanded the adaptive-control network by deploying OPAC at 29 intersections along McMullen Corridor.
Throughout the country, signal-timing plans and existing systems remain unchanged as congestion and urban sprawl grows. Personnel and budget cutbacks and variations in traffic conditions make it impossible to keep up with effective arterial corridor management. While many agencies concentrate their efforts on freeway-management systems, very little has been done to promote intelligent systems on signalized arterial networks.
Through the successful deployment of highly sophisticated adaptive software, Pinellas County has proven the great promise of adaptive-traffic-signal control. Both systems have provided useful data and benefits that will ultimately make adaptive-traffic-signal control the new standard as design considerations are implemented on similar congested signalized arterial systems both in Pinellas County and throughout the world. Any adaptive-signal system should be evaluated before committing to it, especially in regards to the operation and maintenance that will be required for a highly sophisticated system. Given the appropriate technical staff and an eagerness to learn, one cannot deny the benefits adaptive-signal-control software can provide. TM&E
About The Author: Pooran is vice president of engineering and a senior project manager with Telvent North America in Rockville, Md.
