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I grew up in Oxford: a city where everyone cycles. Its roads predate cars, often by a thousand years, so pedal power is a much more sensible way to get around. I say that because I want you to know that this doesn't come from a place of cycle-hatred. I love bikes much more than cars.
Nonetheless, you people are crazy, and the following apparently needs to be said:
The rules of the road apply to you. Really, they do. You have to stop at lights, signal on turns, and obey the general rules of the road that apply to all vehicles. Having a bicycle is not a magical pass to allow you to do anything all over the road. You are not doing two-wheeled parkour.
Don't ride two abreast on a real road. On a cycle path, this is sometimes acceptable, if you're leaving enough room for other bikes to get past. On an actual road that other people use for other purposes, it is never acceptable. Extra bonus unacceptable points for windy country roads that have a cliff on one side. You know exactly who you are.
Lights are nice. You are not a ninja. Ninjas, to the best of my knowledge, do not bicycle while they are ninjaing. Therefore, I recommend wearing lights. These are surprisingly good at helping you not die at night. The red one goes on the back, and the white one faces front. Blinking is probably good, but they don't have to induce some kind of epileptic fit to be effective. You're welcome.
Helmets. Have you heard of them? This doesn't affect me at all. But it might affect you. I know they look sort of lame. But they kind of work when your head smashes against the curb. They've saved the lives of people I know many times. You might want to consider it.
That's it. I don't really understand why Bay Area cyclists are quite so bad. Where I grew up, we had something called a cycling proficiency test - like a driving test for bicycles. It wasn't mandatory, but they did build it into our school curriculum. It might not be a bad idea over here.