Buses and BART remind me that we have a grand civilization. Mass transit is efficient and environmentally friendly and benefits even those who don't use it by reducing traffic. We don't segregate riders by gender or race. As a society, working together through our democracy, we create mass transit systems that benefit everyone. No one person could do it alone.

But it only takes one jerk to ruin 20 people's pleasant rides.

I present to you, in the hopes of mildly improving the mood of the entire Bay Area, a request for better etiquette on BART, Caltrain, buses, trolleys, cable cars and any other means of public transportation.

First of all: "Don't yell." There is a way to use your cell phone politely when other people are trapped next to you. You can make very short, urgent calls while cupping your mouth and the receiver with your hand. "Did Dad get to the hospital all right?" "I'm on the train, so you can pick me up at seven."

If you know reception is bad or your phone won't transmit a quiet voice well, don't make or answer the call.

When other people are so close that they can't escape your voice, do not blather loudly and at length. I should not have to wear earplugs to avoid invading your privacy. Leave me alone.

In fact, "Leave me alone" makes for a fine principle of courteous behavior on mass transit. Don't stare at me or at what I'm reading. Don't play your music or talk so loudly, on the phone or otherwise, that it invades my sonic space. Yes, this includes people with headphones on.

Men, don't spread your legs so far apart that you invade my physical space. If you can't sit without spreading your legs across three seats, then stand instead.

Another humble suggestion: "Get out of my way." On escalators, stand to the right so people can pass on the left. At BART turnstiles and doors, don't block the lines while searching for your ticket; stand aside while you get it out.

On the platform, get out of the way of people trying to exit using the escalators, stairs, turnstiles, or doors. Even if you're not a hurry, someone else is. And if you stand right in front of the bus or train doors as they open, then you slow down the entire system. Stand to one side and you'll get on sooner, which mean the train leaves the station sooner. You'll also incur a smaller quotient of my wrath.

Several similar principles apply: "This is PUBLIC transit," or "do unto others." Other people are using this system too, so check that you aren't making it less useful to them. If it's rush hour, don't take up more space than you must. Move to the end or center of the bus or car, to help reduce crowding at the door. Put your backpack between your legs, instead of wearing it. Let short people have the lower handholds. Let seniors, disabled people and pregnant women sit down, by the door if possible. On rainy days, make sure your umbrellas aren't dripping on another rider.

And now one of the most crucial principles: "I paid as much as you did to ride." Even on crowded buses and trains, some people sit in an aisle seat while leaving the window seat empty, thus challenging other riders to ask to sit down. If there's a more passive-aggressive way to claim unwarranted extra space, I've not heard of it. Why do you deserve two seats instead of one?

Please, fellow riders, don't let these people get away with it. All you need to say is, "Excuse me," as you wriggle into that window seat. The righteousness of our cause will make your rest all the sweeter.

"Excuse me" makes the perfect non-jerky request for a jerk to cease his jerkitude. We can always phrase our requests in polite ways. "Please turn down your music; it hurts my ears." "Could you please make room?"

If the other person reacts with belligerence or you otherwise feel endangered, you can always ask the driver or BART operator to intervene. But usually you'll succeed. The mark of an inconsiderate person is that he's not thinking of the consequences of his actions. It's ditziness or ignorance, not malice. And it's easily corrected.

Don't be afraid to say something if it'll make the ride more comfortable for everyone. We're all in this together.

Sumana Harihareswara's column runs in Bay Area Living on Thursdays. You can e-mail her at sumana@crummy.com.