Before I lived in an actual city, it never occurred to me that running on a sidewalk in a perfectly nice part of town in the middle of the day could potentially be dangerous. But it can. Hazards abound. Ass-hattery is commonplace.
It would be one thing if these hazards were unavoidable (in the same way that, say, bad weather is unavoidable). But they aren't. They are by & large a result of people failing to pay attention to their surroundings and/or take into account how their actions affect other people who are also trying to use the sidewalk.
Now, don't get me wrong. I am not like one of the Tour de France fan boys who ride their bikes on the Golden Gate Bridge walkway at 40 mph & yell at pedestrians to get out of their way like they are somehow entitled to use it as a training course & how *dare* anyone inconvenience them for the half a second it takes them to jump out of the way. When I'm running in crowded parts of the city (usually unavoidable, since I live in one of them), I do my best to pay attention to who is doing what around me and accommodate other people as much as I can, and most of the time the other people around me are lovely, and everything works out fine.
But at other times, some people will insist on making this INCREDIBLY difficult. Let us count the ways.
Friends, I give you: A Full Taxonomy of Urban Sidewalk Douche-Baggery.
Cyclists
I know many thoughtful, law-abiding cyclists. This section is not about them. This section is more about the ass hat minority that seem to lack basic common sense or courtesy for their fellow citizens or both.
If you are over the age of 13, you should never be riding a bike on the sidewalk. Period. In fact, in many areas in San Francisco no one should be riding a bike on the sidewalk. The Valencia Corridor, for example. Yes, it's technically legal for kids, but use some judgment, here, parents. Hipsters are congregating. Activists are activating. Artisinal coffee is being lined up for. Somebody is going to get hurt. I have been three times hit head-on by a cyclist and two of those times, it's because the (adult) cyclist was riding on the sidewalk. Two feet from the bike lane.

I hate you. We all do, actually.
If you're on a bike path that pedestrians share (ie, The Panhandle), a) stay in the right lane, b) use a bell / your voice to warn pedestrians that you're coming up behind them, and c) use a light in the dark. The third time I was hit by a bike, it was because the cyclist was weaving in and out of both lanes in the dark with no light. I tried to get out of her way & it was like she followed me, off the path, right into the grass. There was blood.
Dog Walkers
Again, responsible law-abiding dog walkers, I am not railing at you.
But GODDAMN. There is a limit to how many dogs one person can safely walk on a sidewalk at one time.

Extreeeeeeeeme dog walking!!!!

Livin' la vida loca!!!
Look, People Walking All Teh Dogs: I can't go around you. And neither can anyone else. This sidewalk ain't big enough for the 14 of us.
Extreme dog walking has become such a big problem in San Francisco that we've actually passed a law about it. Look what you made us do.
And it's not just the extremists that cause problems. Urban runners, I'm sure you've met This Guy:
There are places where it's reasonably safe to have your dog on a 30 foot leash. Like in a dog park, or a large open area. But on a busy urban sidewalk, this situation is going to play out one of two ways: 1) Despite my best efforts to navigate it, I trip over your seven hundred foot long leash and go crashing to the pavement, or 2) Your dog decides between my feet looks like a GREAT direction to go and one of us gets brutally kicked in the face. (I am not saying who but one of us is a LOT bigger than the other & wearing shoes.) I have lost count of the number of times this has happened.
I also love how too-long-leash guy is always like, "Oh, sorry! Sorry! Sorry!" Forget your sorries, dude. Sorry doesn't bandage up the bloody spots. Apology NOT accepted.
Children
Children are all brain-damaged. You can tell this because they will see a fast-moving object coming directly at them & just stare at it. I get that; I taught school for eight years. In & of itself, this isn't really a problem. As long as there aren't too many hipsters waiting for coffee or clipboard-wielding Sierra Club members, I can probably just move a foot or so to one side. The trouble is that the all-time favorite activity of children (as far as I can tell) is to move with blinding speed in *completely* unpredictable directions. Sometimes it is like they are actually trying to block your path, while also effecting complete & total obliviousness to your existence. I think I have only actually ever knocked over one small child, but this is only due to preternatural agility & significant gymnastics background on my part.
Look, this ain't suburbia. Busy urban sidewalks are not the place for free-range children. If your child is at or below the brain-damaged age, maybe put them on a leash (not 700 feet long) or in a back pack or in one of those wheeled cart-things. Or, I dunno, pay attention to where they are & what they're doing. That works too.

Yes.

Yes.

Double yes.

Not good very bad abort abort
(You have to imagine the seven hipsters & three Sierra Club ladies.)
Besties
Friendship is just awesome. This is my opinion at all times *except* when I am running alone on a busy urban sidewalk. Then, ooooh, I hate friendship SO EFFING MUCH. So. Much. Because this:

What is your damage?? I know you see me because we made eye contact. I know you know I am moving in your direction. I assume you are capable of comparing the width of the sidewalk to the width of your group. I also assume you don't think I can quantum tunnel through you and/or your besties or collapse all the particles in my body so as to slip through the four-inch gap between you guys and the nearby stoop / storefront / artisan coffee line / gaping whole in the ground.
Don't get me wrong; I love living in the city, but some days it is almost enough to make you move to the 'burbs & scout out some nice, safe fire trails.