I'll not comment on legalities, too many variables.

I'll note a few places I use sidewalks, and some where I wouldn't for anything, but first, my background, since it alters how I ride in the environment.

I'm relatively comfortable in traffic. I'll draft off vehicles, have spent plenty of time riding the shoulder on multi-laned highways, and have blocked my share of narrow climbs. I believe that behaving like a car makes me safer, and think the primary danger to me is drivers who fail to notice me in an appropriate amount of time, whether for physical or psychological/habit pattern reasons. I commute on a SS assault bike and average 16+mph. My previous commute was 4-4.5 miles. I'm starting a new commute that is 8-9 miles, with a possible bus option and a possible bike trail/dirt option that adds 3-5 miles.


Will Use sidewalk: The uphill side of a steep, reasonably lengthy hill on a 4-lane (2x2 plus a turn lane) road posted at 45mph and often driven at over 50. Most importantly, the sidewalk here is wide, separated from the road, and there is very little commercial access/activity (read: no side traffic entering and exiting, no pedestrians). (North Roan, Johnson City TN, S. of Carrol Creek)

Will use sidewalk, but very cautiously: a separated, and minimally signed, MUP adjacent to a 4 lane (2x2 divided) posted at 45 and often driven faster. MUP is short (<1/2 mile) and connected my residence to a feeder street. However, MUP crossed several residential streets, with limited visibility, and well back from the road intersection. This placed the cyclist in a place the driver didn't normally expect traffic from, partially obscured, and placed the automobile in a position blocking the path in normal driving behavior. MUP was also 2-way, meaning sometimes cyclists would cross "wrong-way" such that turning traffic was unlikely to look your way. Horribly designed, but avoided the 18" shoulder on the main road. (Western Blvd, Raleigh, NC, between Jones Franklin and Blue Ridge)

Won't use sidewalk: Commercial area/pedestrian area posted 35 or below. The danger of being off the road, where drivers are not paying attention, is too great.

Won't use sidewalk: 2 lanes my way (or one lane with center turn) operating well below capacity.

Won't use sidewalk: Bike lane or decent shoulder.

Will ride miles out of my way to avoid road or sidewalk: Hilly section of 4-lane (2x2 plus turn lane) posted at 45, often operating near peak capacity (but rarely well over capacity, ie, no excessive slowing). Commercial area with groceries, fast food, and other establishments. There is no safe option. While the lane may be legal, I'd rather spend 20 minutes avoiding it than 5 minutes on it. (Western Blvd, Raleigh NC, between Gorman and I-440)

Far more words than you were probably wanting, but there's my experience. I'm sure plenty folks would do it differently, and some of you probably think I'm wrong, but that's okay.