I usually avoid the sidewalk, except for those times when it's the safest option. Sometimes I ride on suburban arterials, no shoulder, speed limit 40 mph+, and the sidewalk is completely devoid of human habitation. At those times, I don't hesitate to treat the empty 6-ft wide sidewalk as an impromptu bike path. I'm not that interested in mixing it up with high-speed drivers who can't/won't get around me just because I want to prove that cyclists belong on the road.