I wanted exactly this so that I could build some kind of app to let me know roughly what the speed limit of the road I am currently driving is, or at the very least, the speed limit sign I most recently passed. I found "Wikispeedia" as the only one really doing it with any degree of success, but the website is absolute garbage, the demos rarely work, and the API is a bleeding joke. There's not a whole lot to "protect" here. Increase quality (both website and data), then worry about whether or not your data is worth keeping safe from other people.