I've done it when traffic was completely stopped in the LA area, and it's not really that dangerous as long as you're not a dumbass about it. Just like driving a car. A lot of people doing it think that speed limits or things like that don't apply, or they feel that if there's not room it's someone elses fault and get offended at the people in the cars and try to force their way through. You can't say this is dangerous because of people who hit and run, since that happens all the time without this. It's fine to get mad at a hit and run (I know the feeling, it's happened to me) but don't blame the fact that they're riding a bike. Stupid people are everywhere and they cause problems all the time, don't blame one person for someone elses stupidity.