Quick driving thought

This thought came to me this morning while I was driving to work. Their is construction happening on Mather Field Rd (the major street I take to work - also the major street used to access our housing development). They've written the following with spray paint on the ground just before you hit the part of the road they're working on:

Down
Slow

I've never understood why all rules of English language are thrown out when it comes to stuff being written on the street. It's almost as if Cal-Trans (and other companies that work on our roadways) think that for some reason because we're in a car we suddenly stop reading from top to bottom. Another example:

Ahead
Stop

Make sense to anyone else?