You really need to do something with Garmin.
http://developer.garmin.com/web-products/web-services/ for example
They have all that data already.
My Garmin Nuvi 205 tells me where I am and what the speed limit is there, so Garmin has the data.
You can certainly get a small GPS and have it provide GPS coordinates to the arduimo
http://www.mdfly.com/index.php?main_page=index&cPath=8_136
but recreating all the map data, that's a lot of work.
Are you there now? Drive the route(s), capturing the where location & the speed limit as you see changes.
I know the Garmin GPSs will let you do a demo mode too, I don't know if speed limits are shown along the route.
I tried getting coordinates from maps.google.com, don't see a way to do it. Am thinking they hid that stuff, thought it used to be shown.