SpotScout Inc., a Cambridge-based Web startup, plans to create an online marketplace where drivers can reserve private spaces in garages and driveways and swap public parking spots via mobile phone, laptop or PDA, The Associated Press reports. Drivers will be able to post information about their planned departure times and offer both public and private spaces for a price. Owners of garage and residential driveway spaces will offer reservations by posting information about times when spots will be empty—a process the company calls "SpotCasting." All payments will be made electronically with credit cards or with the online payment service PayPal. SpotScout plans to keep a 15 percent cut of garage and private parking space transactions, and is still working out how it will make money from auctions of public spots. SpotScout plans to begin offering test versions of the service this spring in Boston, New York and San Francisco, with eventual rollouts planned in other large cities. The auction system for on-street public spots won't be introduced until next year. Some analysts question whether SpotScout can make online parking searches quick and easy enough to win over a critical mass of consumers, especially since only about 10 percent of mobile phone owners using the devices for Web access, according to The Associated Press. In addition, city officials worry about the prospect of citizens trafficking information about public spots that are supposed to be available first-come, first-serve and can't be reserved.