This invention localizes traffic condition detection and classification to a vehicle. Vehicles work cooperatively to fuse their traffic condition assessments so as to produce larger geographical coverage and more reliable evidence of the conditions. The system can be executed on an onboard device which includes at least one of the following: GPS capabilities, connection connected to the vehicle to measure vehicle speed, or communication with a cell phone. In the example of the cell phone, speed can be computed from phone GPS, a GPRS/CDMA, or both. Otherwise, vehicle speed can be determined obtained from in-vehicle on-board diagnostic system (e.g. using the OBD-II protocol) or based on GPS and accelerometer readings.