What Speed?

Now , as a few of you may know...I have had a little "problem" with speed recently...... So..maybe this is why this caught my attention!
WHAT kind of speed restriction is this....?
If you drive at 4 and 3/4 miles an hour... will you be transported to Platform 9 and 3/4's and subsequently end up in Hogwarts?
Hmmm...!