Abstract: Methods are introduced for determining user activity states (walking, driving, biking, jogging, etc.) using smartphone IMU and other sensors. By use of such states, a novel parking spot sharing system is implemented; walking after driving is interpreted as necessarily involving parking. Parking spots are thus recognized, and users walking towards their parked cars are propitiously prompted to share their soon-to-be-vacated spot with other users of the system desiring to park, for credit towards similar services in the future, prizes, money or other incentive.