I witnessed a bike crash just last week (in Boston) where the convoy was biking (around ~30 kph I think) and some asshole opened his car's door without looking his surroundings, and the biker in the front bumped into the door and flew over her car. Luckily, the road was clear (so she wasn't crushed by a car) and she had a helmet. She was fine but she had a concussion. Clearly, if we were going at 25, the likelihood of she dying would be even lower, and if we were going 50, it would be significantly higher (her bike could break door's glass and she could have permanently injured herself).

I don't understand this whole thread. If you're biking in a city like Boston or SF early in the morning or around 5pm, you're trying to go office/home. What's the point of biking at 50 kph? This is not recreation. This is something I do every day and my highest priority is doing this as safe as possible. Similarly, would you drive significantly faster than speed limit with your car, because you arbitrarily think "it's not dangerous"? It's nice that you think bumping into things at 30/50 is not dangerous, I respectfully disagree and consider it absolutely dangerous and kindly invite you to stick to whatever speed limit you have in your city. People are just trying to get to their work or kids and be done with their days, please don't try to be Superman.