Nice idea, but it relies on data
This is a fair enough idea, but there's a fly in the ointment.
I bought a brand new car just over a year ago, and it's full of cool tech. One of the many features is that it shows me the speed limit right next to my current speed, and the icon changes if I'm speeding.
Happily there's no audible warning ... because the speed limit it shows is sometimes wrong. Only this morning I was going along a road with a 30mph limit, and it was telling me the limit was 20. I had the same in a 40mph limit not so long ago.
So fair idea, but if it's to work they need to sort out the tech (I suspect it might be a data issue rather than a geolocation problem, as the position on the map display is right).