The present invention provides techniques whereby objects are readily identified. These inventive techniques are ideally suited for traffic monitoring. In one implementation a vehicle is marked with a machine-readable code on an outer, top surface of the vehicle. An elevated or aerial platform captures an image of the vehicle. A detector analyzes the captured image to recover the machine-readable code. The vehicle is identified from such. Traffic congestion can be determined by monitoring the movement of a marked vehicle through the city traffic. In some implementations the machine-readable code comprises an identifier, which is used to interrogate a database for related information.