Most turtles live in wetlands, but they lay their eggs in nearby upland areas. I suspect the snapper was scouting out a nesting spot where it is high and dry near our office, but actually lives in one of the wetlands across the street.
No one explains to the turtles that they need to stay away from roads because of the cars. I would imagine that for them, the road is just a really smooth and easy strip of land to cross while they are scouting for a good spot to lay their eggs.
The next challenge comes when the baby turtles (about the size of a half-dollar) hatch from the eggs, and then through some homing instinct, head off to find their mothers. These little guys are easy prey for many animals and birds. And if they have to cross a road, they are obviously very vulnerable to being squished -- let's face it, they are really small (so not very visible) and they are really slow (because their legs are so tiny!).
So, I'm thinking some well-placed turtle crossing signs are in order to warn folks to be on the look-out!