Traffic signs and traffic lights mounted on posts close to a lane will appear in a roughly known region of an image recorded by a vehicle-mounted, forward-looking camera. Since traffic lights and signs appear in shapes and colors known a priori, color cues are exploited within such search regions to quickly collect subregions which should be tested for compatibility with the hypothesis of representing a traffic sign or traffic light. Approaches which rely only on shape cues in order to detect and classify traffic signs seem to be less reliable.
It appears as if the `quick-and-dirty' approaches towards traffic sign detection and recognition have been exhausted. The challenge to gradually decrease the failure and false alarm rates as well as to increase the correct recognition rate even under adverse conditions is likely to be taken up by specialized research groups within companies. Relevant results thus are less likely to be described in detail in the scientific literature.
The norms regarding shape and color coding within traffic signs support a systematic approach towards their detection and classification. In order to obtain reliable detection and classification results, however, significant efforts are required even for isolated traffic signs positioned along highways and rural roads. Raw color data have to be carefully postprocessed in order to delimit detrimental influences of illumination or recording conditions. A multi-step hierarchical approach, intermixing color as well as shape evaluation in the image plane, appears to be necessary just in order to classify an image region into one of the many categories of traffic signs. Given a 1996 vintage VLSI CPU, this is possible in real time provided there are not too many distracting similar signs or shapes. When it comes to a reliable deciphering of symbols within a traffic sign, less is known.
Traffic sign recognition on highways is usually attempted with a tele-camera (fixed to the vehicle) in order to facilitate early detection. Not much has been published about repeated evaluation of the same signs in consecutive image frames, nor about combined evaluation of traffic sign images in windows extracted initially from sequences recorded by a tele-camera, with a subsequent switch to image regions from frames recorded by a wide-angle lens camera as soon as the vehicle approaches the traffic sign. Only cursory reports are available regarding advantages to actually track the hypothetical image of a traffic sign by a video-camera mounted on a pan/tilt-head within a vehicle in order to increase the signal-to-noise ratio for a more precise evaluation.