When buying petrol read the signs. A few states, including Oregon, have laws that forbid you to pump your own gas. The line markings on streets can be confusing for Australians. A solid white line across the middle of the road is not necessarily associated with a stop sign. Unless there is an accompanying Stop sign treat these as though they were pedestrian crossings.
Roundabouts are pretty rare.
In their place are 4-way Stop signs. I'm told Washington DC though has many roundabouts. I am orignally from California and you're right there aren't any out there. I now live in New Hampshire, and there are lots of these in New England though here they are called rotaries . It really doesn't matter what side of the road you approach them from, they're still a pain in the ass.