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Wire loop detectors are embedded in the freeway pavement and at ramp metering lights on the freeway system.

Smart Technology for Congestion Relief
by  Maria Raptis
Issue Date: 04/2011

Traffic management helps the existing freeway system work smarter and more efficiently to promote smooth, safe and consistent traffic flow.

You’re in bottleneck traffic on the freeway. Maybe there’s an incident or a lane closed up ahead. Seeing neither, you wonder what’s going on? It’s the basic economic concept of supply and demand. In terms of congestion management, bottlenecks occur when traffic demands exceed the available roadway space, or freeway capacity.

For decades, Caltrans has implemented several technological strategies for congestion relief and to provide measurable benefits to the transportation network and to the public. District 7 uses technology in various ways to better manage freeway congestion and to better communicate real time traffic situations to the travelling public.

Changeable Message Signs (CMS), strategically placed along freeways, provide real time information on travel times and traffic conditions. Motorists can benefit from CMS information to take alternate routes, plan daily routes, avoid delays and minimize congestion. Ramp and connector meters regulate the flow of traffic entering freeways and freeway cameras allow traffic managers to quickly investigate, identify and respond to traffic incidents to clear the roadway.

But, it is the wire loop detector that has been around the longest – over 40 years - and they are the backbone for traffic management and data collection. Caltrans collects traffic data from wire sensors called ‘loop detectors’ that are embedded in roadways usually at half-mile intervals. With as many as 25,000 or more loop detectors on California’s state highway system (as of 2009), the sensors provide real-time freeway data every 30 seconds. District 7 has the largest sensor network in the state with more than 8,700 loop sensors.

A wire loop is activated by an electronic signal ranging from a 10 to 200 kilohertz frequency, which creates an electromagnetic field. When a vehicle passes over the loop or stops on top of it, the vehicle's undercarriage acts as a conductor that interferes with the electromagnetic field. Electronic measurements, communication and various mathematical algorithms are used to generate data and information about traffic flow volumes and occupancy, speeds, travel times, and the approximate location of an incident.

The wire loops track travel time information and speeds that reflect real-time traffic conditions. The data collected from the wire loops can be translated in many ways. For instance, the count for how many vehicles pass over each loop at any given time determines traffic volume. The length of time that a vehicle remains on the loop indicates a possible incident, congestion or if traffic is flowing smoothly. Even the length of the vehicle can be calculated to show if the vehicle is a car or a truck.

Motorists can see a circle outline cut into the pavement to mark the spot where a loop of wire has been inserted. They can be placed up to 20 inches below the surface of the pavement. Though the loop detectors are barely more than metal detectors, they collect enough information to indicate the general speed of traffic. When a vehicle passes over the loop, a signal travels to a vehicle detector station (VDS), or a ‘pull box,’ at the side of the road.

One pull box can control about 40 wire loops. For the public’s benefit, this data is relayed into a graphical display using red dots to indicate slow speeds, yellow dots for heavy traffic and green dots for smooth flowing traffic areas. The public frequently sees these colored codes in media traffic maps and reports in newspapers, websites and on television.

The Caltrans freeway sensor network has two components, a sensor system and a communication network divided into twelve parts, each built, operated and maintained by one of the department’s twelve districts. Statewide, over 8,000 VDS collect the data from the information relayed by the radios in the sensors and deliver it via a cellular communication network. In District 7 that central hub is the Los Angeles Regional Transportation Management Center (LARTMC), where computers receive real-time traffic flow data relayed from the loops. The loop detector technology and many other smart technological tools help Caltrans to minimize congestion and to identify roadway incidents and clear them as quickly as possible.

Businesses gauge the price of a product on its supply and demand. With the freeway transportation system, particularly in the Los Angeles region, the supply is set. There is only so much space available but the demand increases as more vehicles are added to the freeway system. To move people, vehicles and goods quickly, reliably and safely, traffic managers look towards newer and smarter technology to better manage the demand and harmonize traffic flow.

"Real-time and accurate measurements of what is happening on roads are crucial for the future of traffic and congestion management, said Frank Quon, District 7 Deputy Director, Division of Operations. “Economically, congestion has a price that is reflected in delay costs and passed on to consumers.”

Quon emphasizes that Caltrans distributes and shares with other entities accurate freeway condition information so that motorists can do their part to relieve congestion by making well-informed decisions about their commutes, follow CMS information, travel during off-peak hours, use public transit or take advantage of traveler information phone services offered by dialing ‘511’ in southern California or subscribing to other traveler related services offered by internet service providers.

“Mobility is vital to the local and state economy. California citizens recognize the importance of investing in transportation infrastructure to help reduce congestion and meet the state’s transportation needs,” said Quon. “Caltrans will work diligently to get the most out of its available resources and invest in innovative congestion relief strategies to get as much service as possible out of our existing freeways and highways.”
 

Caltrans traffic and ramp metering engineer inspects the electrical box at a Los Angeles on-ramp. The public regularly sees real-time maps of current traffic conditions in the media. Travel time data is fed from wire loop detectors and  green indicates free-flow freeway conditions.