I've read that you should never relocate a turtle. Many turtles have "Home Ranges", a territory they call home, and when relocated, they will search out ways back. Besides risking many additional road crossings, some turtles, if they cannot find their way back will stop eating and just wander listlessly. Also, if the turtle is carrying a disease it can devistate the "local" turtle population.
They are usually heading to nesting grounds when seen on roads. It is great to help a turtle across the road and not a bad idea to wait a few minutes to make sure it carries on in that direction and doesn't turn around to cross the road again.