Yikes! Hope you're relatively unscathed.

Happened to me years ago in city traffic I was more or less pacing. LH crank arm broke and my clipped in shoe and pedal scraped on the pavement while I somehow remained upright and worked my way over to the RH lane without being run over.

Aluminum gives no warning, just fails. Were there telltale cracks forming beforehand? I'll never know.