It is the urban driver"s most agonizing everyday experience: the search for an empty parking place. Circling, narrowly missing a spot, cleverly defeating other motorists to finally【C1】______into a space only to discover that it is【C2】______limits during working hours. In San Francisco, it is also a worrying traffic problem. Drivers【C3】______for parking spots generate 30 percent of all【C4】______traffic jam, city officials estimate. Now San Francisco proclaims that they have found a【C5】______—a phone app for spot-seekers that【C6】______information about areas with available spaces. The system, introduced last month, relies on wireless sensors【C7】______in streets and city garages that can tell【C8】______seconds if a spot has opened up.【C9】______the system could come with serious consequences. Safety advocates say that drivers on the search for parking could【C10】______focusing on their phones, not the road. "It could be really【C11】______," said Daniel Simons, a professor of psychology at the University of Illinois, where he studies the science of attention. And, he said, it could also be【C12】______: "Most people are looking for parking spaces in places that have a lot of traffic and a lot of【C13】______." City officials acknowledge the potential【C14】______. They are urging drivers to【C15】______before they pull up the city"s iPhone app, or to do so before they leave home. But the spots can disappear quickly,【C16】______any circling driver knows, and for plugged-in motorists in the habit of texting or glancing at the GPS, the urge to use the parking app is certain to【C17】______as the frustration does. San Francisco has put sensors into 7,000 metered parking spots and 12,250 spots in city garages. If spaces in an area open up, the sensors communicate wirelessly with computers that in turn make the information【C18】______to app users within a minute. On the app, a map shows which blocks have lots of places (blue) and which are【C19】______(red). San Francisco"s is by far the most widespread【C20】______that several cities, universities and private parking garages are experimenting with.