Both.

I've never used mine to navigate on the bici, but I've used it a couple of times in the car. The user interface is essentially the same as the Garmin car GPS's my DH uses. You have the option to set it to navigate by car, bicycle or walking; and under that, you can choose to avoid unpaved roads (not entirely accurate, but some help), toll roads and/or limited access highways. (obviously the bicycling and pedestrian options eliminate limited access roads by default).

Mostly I use it the other way, to guide me along a route I or someone else has already mapped on mapmyride or ridewithgps.

Either way, I would hate to be without it.

But I don't know what the 605 does that the 500 doesn't?