https://support.garmin.com/en-US/?faq=S ... PRe0WgzrW7Unfortunately this is how the device is set up to work because the TopoActive mapping is based on OSM (Open Street Mapping) that has not been verified safe for Automotive Driving. The problem is with the type of mapping it comes with. You can always load another type of mapping that is certified safe such as City Navigator.
Changing the Activity type to Hiking, Cycling, or just about anything else allows routing to follow roads and trails on the built-in maps that come with the 66i.