By far, the scariest roads to me are "fast" suburban 4-lane streets with no shoulders and high curbs. Drivers are packed in on them, and are typically doing 5 to 15 mph over the speed limit. So much traffic they can't see past the next car so they don't see you until they are up on you, and they don't have room to swing out to avoid you.
I got brushed on the elbow by a FedEx truck two or three weeks ago on a road like that. Not good.
Highways rarely bother me as much.