I stumbled upon a website called www.wikispeedia.org which aims to store every speed limit sign in the world online. They do this thanks to users who submit data through a smart phone app as they find them. The app links their current location to the speed sign value, which is then stored in the database.
I'm in the process of making my own app to do this; screenshots below. Later on I'm planning on using their database for an app I have in mind. I should have the app finished in a few hours' time, hopefully sometime tomorrow.