In many urban areas, a third of the cars on the road have already reached their destination and are just circling the block waiting for a parking space. This leads to a cascade of problems, including pollution, traffic congestion, and accidents. Now massive arrays of networked sensors installed in city streets could significantly improve the situation by helping drivers find parking spots quickly.

"We're trying to make it really easy for people to find a parking space," says Jay Primus, who manages San Francisco's new SFPark program, the most advanced smart parking system in the United States. The city has recently installed magnetic sensors into the asphalt beneath 8,200 street parking spaces and is also collecting information on thousands more parking spots in garages, as well as from smart parking meters. All that information is linked to a central management system. Drivers can use a website or smart-phone app to access real-time data about where parking is available and how much it costs.

"Circling drivers are distracted drivers," Primus says. "They're much more likely to hit pedestrians, bicyclists, and other cars, and as they search for parking spots, making frequent turns and making frequent stops, they can cause unpredictable delays to the transit system." He says the city hopes to reduce carbon dioxide emissions and collisions, improve transit reliability, and help businesses attract customers. The project is funded by a $19.8 million grant from the U.S. Department of Transportation.

In Los Angeles, researchers once found that cars circling for just three minutes to find parking in a 15-block neighborhood racked up over 350,000 miles of excess driving every year. Now L.A. is building a smart parking system similar to San Francisco's. Within the next few months, the city will have sensors at about 7,000 street-side parking spots. "If you count the parking spaces, L.A. doesn't have a shortage of parking spaces," says Peer Ghent, a senior management analyst at the Los Angeles Department of Transportation. Instead, the problem is that drivers don't know where the open spaces are. That's an issue the smart parking system could fix with its network, which also allows cities to quickly see which cars have exceeded their time limit or are parked in front of fire hydrants and bus stops. In L.A., police officers can use a phone app to identify cars that are in violation, so they don't have to check every car on the street.

Real-time data about parking-space occupancy allows city managers to set parking prices to encourage drivers to park in less congested areas. Since April, when the system went live in San Francisco, managers have changed the pricing structure three times. On one busy street in the Fisherman's Wharf district, it costs $3 an hour to park; turn the corner and the price drops to $1.50.