On the last Friday of every month, hundreds, sometimes thousands of cyclists take over the streets of San Francisco after gathering around the Embarcadero Plaza.
The bicycles range from regular road and mountain bikes to tall, upside-down, back-to-front contraptions. Most people wear costumes or carry along signs and props.
I snapped this shot over my shoulder without looking backwards to avoid crashing into the mass of wheels (Wikipedia - Critical Mass.) all around me in Market Street.
Critical Mass doesn’t only happen in San Francisco—it can happen anywhere, so try and organize one where you live. Here’s a video of one in New York (Jay’s Movies - Critical Mass.).