If at all possible I try to stay off of busy highways. There is just too much unpredictability there. Back roads take longer but are a little more sane. When I find myself on a busy highway I found the middle lane is a good lane to travel in. This way you can pass if you want to but not have to be concerned with people merging in front of you from the on ramp as is the case if you ride in the slow lane. In the slow lane, at every on ramp people are looking to merge and you are working a bit harder in maintaining your spacing.