Local Image Features Extraction for Road Signs
Do you know what signs you are looking for? If yes, maybe you could do template matching (e.g. a normalized cross-correlation, available in matlab). It won't work great when signals are getting closer, since the perspective projection will change their appearance, but it should work for mid-range detection.

You can limit your template-matching search to the right side of the road.

There are also template matching algorithms for affine transformations that might be a next step.

Christos