How to detect road sign without color information???

This is one of the video frames that I recorded. I do project that detects road sign and track it in video file. What I need to consider is

1. It is saved as grayscale, so I can not use color information

2. The traffic sign is rectangular shaped and it has fixed aspect ratio

3. There are special signs in the traffic sign, and it looks like arrow "<", ">"

4. The video was recorded in various situations, so sometimes the traffic signs is really dark. So, I think using bright information(the values in grayscale image) would be difficult..

5. The letters in the traffic sign are various, so just training the entire traffic sign might not work..

I tried many ways... such as Hough, Applying filters that detect brighter part than local neighborhood... I think detecting rectanular shapes should be done ahead and then check for the special sign ">" and "<". Any ideas should be welcome. Thx in advance!!

[Walter Roberson]

It does not appear to me that the aspect ratio is fixed. The sign size appears to vary to hold the amount of information needed, with the height of the various elements probably being fixed (fixed font size for example.) That would lead to varying aspect ratios.