I have to admit to being a bit peeved that I have not heard of this study earlier...
Don't be. This study / experiment was debunked. About a week after this 'big announcement' in June.
Turns out that the 'scientists' didn't calibrate their instruments -- Oops. ;-)
So 'c' is still 186,000 mps (approx). And no new 'Speed Limit' signs need to be made ;-)