Wikispeedia is a website trying to gather all speed limit signs on Earth. I recently created a Google Visualization for some of their data, specifically on speed limit signs that change speed throughout the day.
Check it out here: http://groups.google.com/group/wikispeedia/browse_thread/thread/c9c712125a597b16
Here is how to see and comment on what they are doing:
Google groups: http://groups.google.com/group/wikispeedia?lnk=