Adaptive Traffic Control Systems

Poor traffic signal timing contributes to traffic congestion and delay. Conventional signal systems use pre-programmed, daily signal timing schedules. Adaptive signal control technology adjusts the timing of red, yellow and green lights to accommodate changing traffic patterns and ease traffic congestion. The main benefits of adaptive signal control technology over conventional signal systems are that it can:

• Continuously distribute green light time equitably for all traffic movements
• Improve travel time reliability by progressively moving vehicles through green lights
• Reduce congestion by creating smoother flow
• Prolong the effectiveness of traffic signal timing

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 Technologies (ASCT), in conjunction with well engineered signal timing, can do just that. 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 our local roads and highways. Less unnecessary delays and traffic moves quickly and smoothly.

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 percent. In areas with particularly outdated signal timing, improvements can be 50 percent or more.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, frustrated drivers, and degraded 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 continually.

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. Other disruptions, such as crashes, are impossible for time-of-day signal timing to accommodate.

Cutting Costs

Outdated traffic signal timing incurs substantial costs to businesses and consumers. They account for more than 10 percent of all traffic delay and congestion on major routes alone. For consumers, this causes excess delays and fuel consumption. For businesses, it decreases productivity and increases labor costs.

According to the Texas Transportation Institute, the cost of traffic congestion is $87.2 billion in wasted fuel and lost productivity. That translates to $750 per traveler.

Outdated signals also affect State DOT costs. Personnel must respond to citizen complaints when traffic signals do not meet traveler needs. Personnel compile the data for transportation specialists who then analyze the data and develop updated signal timing using the traditional signal timing process before generating their recommendations. Because these specialists must balance the needs of one intersection against system requirements, this is time consuming as well as expensive.

With ASCT, the data collection and analysis are done automatically. More important for travelers, signal timing updates are made as situations occur—topping many complaints from ever happening.

Types of ASCT

Implementing ASCT will maximize the capacity of existing systems, ultimately reducing costs for both system users and operating agencies.

Many 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" and ACS Lite.

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.

Customer Satisfaction

With ASCT, the FHWA addresses a legitimate problem. The 2007 Traffic Signal Report Card, released by the National Transportation Operations Coalition (NTOC), assigned an overall grade of "D" to traffic signal operations practices in the United States, indicating that "agency programs that support efficient maintenance and operations of traffic signals are not as effective as they could be."

On average, travelers spend 36 hours per year in traffic tieups. For urbanites, the number is much higher. Collectively, Americans spend nearly 4.2 billion hours sitting in backups. Implementing ASCT can help improve customer satisfaction scores.

Other Benefits

Adaptive signal control technologies are also kinder to the environment. Using ASCT can reduce emissions of hydrocarbons and carbon monoxide due to improved traffic flow.

Real-time management of traffic systems is proven to work, yet these systems have been deployed on less than 1 percent of existing traffic signals. FHWA is now working to bring these technologies to the rest of the country. For frustrated travelers, the optimal balance of red light/green light is on the way.