I fell while going downhill at about 20 mph. I was looking back in preparation for a left turn so I was surprised and don't know exactly what happened. I reconstruct on the basis of the aftermath; I haven't reconciled the data.

The left side of the handlebar is significantly bent in (but still usable). The cover of the left brake lever was torn off and the lever dented (but still usable). The glove (a winter glove, not a bicycle glove) of my left hand was torn up. I got 22 stitches on the left side of my face; I broke the eighth rib on the left side. I had a bad bruise on my left leg where the pedal would have hit it.

The deepest wound was in my right elbow (3 stitches), but the right handlebar, brake lever, and hand weren't badly injured. The right pedal broke (the outside of the cage); the rear derailleur and hanger (and, perhaps, the dropout) bent.

I started on the right side of the lane; when I picked myself up I was closer to the left side. The damage to my body is most consistent with falling onto my left side; there's more damage to the right side of the bike. I don't remember which side I was lying on when I stood up; I was still on my bicycle (I think.)

Could I have bounced? Occasionally my left arm enervates for a fraction of a second: if that happened, I would have swerved to my left and fallen on my right side. If I unconsciously turned right to compensate for turning my head to the left, a bad habit, I would have fallen on my left side. Neither explains the damage on both sides.

I lean to falling on my right side, hitting on the elbow, pedal, and derailleur, then bouncing up and over to end on my left side. Does this seem plausible?

The wheels and fork were unharmed.

sorry you are hurt , hope you heal soon . as far the bike goes replace any broken parts . don't worry about what happen , just get better and start riding soon .

Of course you could have bounced. At that speed, many things could have occurred. How about this scenario? You fell to the right, taking the hit on your elbow, and did and endover, only sideways. And then slid along the pavement, damaging both your bike and your body on the right side. There would be no damage to the wheels.
Get well soon.

Yes. You could have slid out, falling on your right side, then popped up enough to regain traction, flinging you to the left. Google "motorcycle high side" for some graphic video.

The injuries are consistent with this: lacerations on the right where you slid, trauma on the left where you landed from a height.

Sorry about the injuries. The skin injuries will heal pretty quickly, but broken ribs suck.

What's your right foot and shoe look like? THat could easily be what took out the rear der.

My quick theory is you did a simple, but hard slip down on the left side. Caught yourself on the destroyed left glove enough to rotate your torso clockwise tearing your right foot out of the pedal out and down into the rear der and landing on your back with the exception of your injured right elbow catching you. It's a theory, but consistent with the parts I break when I crash.

Given that you weren't attempting a high speed turn where the rear wheel slides out, I suspect a high side fall, though different from what Jeff W. described. In this scenario you unconsciously started a turn without preparing for it. This would bring the bike to the inside of the turn and you to keep going straight as the bike turns out from under you. Once the process starts, new steering forces come into play and the front wheel bites throwing you to the ground with a bit of energy.

These often have a bit of a bounce component to them, sort of like an angled endo, or if the bike sustained damage only on one side, you could have slid and rolled accounting for injuries on both sides.

20mph is enough speed for multiple bounce and rolls before you come to a stop.

Glad to hear you're OK, and walking (limping) away with comparatively minor injuries which will heal.

this was about 0.5MPH.

Wow that's a tough fall, jeeze that must've hurt. I took a bad fall late last summer myself but compared to yours, it was a 'good' fall. Hope you get better quick.

Please do yourself a favor, bring your bike to an LBS for a going over of frame and other damage. Paint, cracks, welds alignment etc. A trained eye can spot stuff, as well as measure things out of alignment.

Glad you're well enough to type!

Piece of advice about broken ribs, be sure to regularly, consciously breathe deeply, even though it hurts. The first time I broke a rib, I avoided deep breathing because of the pain, and developed pneumonia because my lungs were not getting properly aired out.

I've only fallen once while the bike was moving. I glanced backward to see if my wife was lagging behind and lost my balance for a split second.

My wife also fell recently. Same scenario: she looked back to see if I was still following her. Neither of us was using a mirror when we crashed.

My belief is that looking backward can affect your sense of balance, resulting in a spill. If you think about how amazing it is that we can balance, pedal, and turn a rapidly moving bicycle, it's quite a remarkable feat, even while looking forward. A backward glance can be enough to upset this delicate applecart.

If you don't already have a mirror, it would be a great investment. If you find it awkward to use, keep trying until you are comfortable with it. With very slight side-to-side head motions, a helmet mirror can "pan" the entire width of the road behind you.

Nice picture!

I started using a good sized rear view mirror mounted to the brake hood to reduce the need to look over my shoulder all of the time.