Most cyclists, myself included, advise people not to ride on the sidewalk. There is some evidence that sidewalk cycling is more dangerous than riding in the street. Some might even say the evidence is overwhelming. But let us not throw the baby out with the bathwater. Sidewalk cycling is (IMO) more dangerous than on-road cycling, but at times it is appropriate and a useful tool in the cyclist toolkit.
I always compare sidewalk cycling to SCUBA diving. Just as SCUBA diving is more dangerous than swimming, it does not mean it can not be done safely - it just requires more caution. it's not that you can't ride safely on the sidewalk it's that you can't ride on the sidewalk as you would in the street and be safe. You have to slow down - often to walking speed, and you have to be extra careful at the intersection and curb cuts. If you act like people can't see you when road cycling, act like they are trying to kill you a.la Death Race 2000 when you're on the sidewalk. You have to be on the lookout for added hazards like cracked pavement, sidewalk furniture, low branches etc.. And of course you have to be cognizant of and courteous to pedestrians. You don't like it when cars fly past you with a two inch passing distance and pedestrians don't like it when cyclists do it to them on the sidewalk. Mrs. Washcycle recently became a pedestrian commuter. She loves bikes - but she does not love them flying along the busy sidewalks downtown at 20 mph and scaring the bejeezus out of her.
Having said that, here are some good things about sidewalk cycling
Sidewalks make good climbing lanes: As part of my daily commute I use sidewalks both going to and coming from work. Both situations are almost identical: I'm going up a steep hill, the outside lane is narrow, the sidewalk is wide, traffic goes fast, there are few curb cuts and no pedestrians. Since I'm going 7 mph I don't want people barreling at me at 45 mph.
Sidewalks make good contraflow lanes: When people recently got tickets for going the wrong way on New Hampshire many suggested they just use the sidewalk. Now going downhill on a sidewalk sucks because you can't fly down the hill (again with the slow) but sometimes, even with the speed constraints, going the wrong way slowly for one block is better than going far out of one's way.
Sidewalks can be adequate bike lanes for roads with a low Bicycle LOS: Usually I would say you're better off in the street, but there are few roads where traffic is so fast and lanes are so narrow that even though you aren't going uphill you might be better off on the sidewalk- like on New York Avenue NE or Irving Road near the Washington Hospital Center.
Sidewalks can be passing lanes: Again, not recommended very often, but when on-road traffic is a real mess (not moving at all and too crowded for lane splitting), the sidewalk can serve as a good way to get around it. In downtown I once found I was better off walking my bike on the sidewalk around one such cluster than trying to negotiate my way through it.
You're mileage may vary, maybe you think no one should ride on the sidewalk ever. That's fine. But it worries me that so many people are so quick to denounce sidewalk cycling at all times. It's the kind of attitude that gets bad laws passed.
Speaking of sidewalks, one of the more unusual things I carry in my pannier is a pair of cheap garden shears. When plants start to grow over a sidewalk or trail I know that maintenance could be months away, so I pull over and give it a few snips - very satisfying. I figured I was the only one. Seems I'm not even the only on in DC - Stephanie Kay at WAMU does it too. Here's a response from the gardeners (I would never cut a homeowner's plants without talking to them).
When biking on the sidewalk it's a good idea to avoid the more constricted sidewalks in DC.
Photo by Kavitha Cardoza