Limitations of Traffic Sign Recognition

Limitations Of Traffic Sign Recognition

Supplementary warning symbols on speed limit signs
The only supplementary warning symbols that the system can recognize and interpret are those for wet roads, rain, snow and lorries. Other supplementary warning symbols are not recognized. The speed limit is displayed without any indication of a supplementary symbol. The supplementary warning symbol for wet roads appears only when the windscreen wipers are in operation. Speed limits that only apply at specific times of day are identified based on data from the GPS navigation system.

Parallel roads, splits and converging streets
When streets adjacent to the street on which the vehicle is currently travelling also have speed limit signs, it is possible that the system will respond to the speed limit sign on the adjacent street.

Signs on lorries
The signs mounted on lorries can trigger detection errors in the system. This malfunction is usually triggered by stationary lorries, during overtaking maneuvers and when the vehicle is underway behind a lorry.

City and town limits
The speed limits applicable within city and town limits are taken from the data in the GPS navigation system. If these data are not current, spurious warnings may be generated.

Partially obscured and dirt-covered signs
The system may be unable to recognize signs that are partially covered by trees, dirt, snow or lorries. Problems can also arise in response to heavy rainfall, fog and when the sun is low in the sky.

Non-standard signs
Signs must conform to the standard format for effective recognition. Information signs can also interfere with the system's ability to interpret limit signs. The system is not able to distinguish between information signs and speed limit signs. For example:
- Information signs at border crossings
- Signs for suggested maximum speed
- Military notice signs
- Numbers on motorway exit signs
- Maximum load ratings posted at bridges
- Signs indicating the end of a no passing zone

Speed and distance
At high speeds and when a sign is still at a considerable distance from the vehicle the system may exhibit a sporadic inability to recognize the sign. As these two variables become more pronounced, so does the probability of failure to recognize signs. The effect is more pronounced during night-time driving.