Many posts here are soliciting information, recommendations, etc. This one is more a statement of one experience from last week.
Riding my normal daily route two of us had stopped at a stop sign, waited for traffic, then departed onto a busy street. I pedaled two or three strokes then stood up in a tall gear and hammered to accelerate to around 20 mph. Nothing unusual at all. A clear, beautiful morning the same routine that I have had 3-4 days each week for 10 years. All I remember is thinking, " this is going to hurt," as I slammed my head into the pavement. The impact knocked me out for awhile and my riding partner who heard the impact came back and got me and my bike up out of the traffic. After several days I have pieced together what I think happened. My right shoe came unclipped on the downstroke and the shoe hit the pavement with the toe bent underneath, the chainring impaled itself in the back of my leg. I am not sure of the physics of the rest, only that my helmet (the styrofoam) broke through at impact, and I left a pound or more of flesh at the scene.
I am not trying to sell helmets, nor do I care if you wear a helment. If I had not had a helmet on, my cranium would have fractured from this impact. The helmet left a large bruise on my forhead as it was. Two days later I went to play tennis and discovered that my right arm was not functioning properly. The impact had put a lot of pressure on my neck and spinal cord which caused swelling etc. even though it wasn't really painful. My arm has come right by now after several days on the tennis court. I have not been on a bicycle for a week.
Just saying, be careful out there.