White Road Sign Detection
Your question is broadly answered here: How to extract traffic signs from a photograph?

Your specific case is a little more complicated because the signs are not particularly distinguishing, however the same suggested techniques of cross correlating the images with clean images of original signs (you will probably want a library of images containing all signs for best results) are still applicable.

One suggestion above was to use an infrared camera which will be very effective if the road signs are micro prismatic or glass bead designs.