This probably ins't so much specific to Seattle, however I never rode the bus in L.A., so it is strange to me.
Now, like men have an unwritten code for urinals, there is an unwritten code for bus riding. Bus etiquette is actually quite similar to urinal etiquette coincidentally, so I find that men don't have quite as hard of a time with it. It's not a hard code to pick up on; or so you would think.
1. Don't sit right next to me if there are other open double seats. There really is no reason to do this. Everyone likes their personal space, so why voluntarily invade the bubble (which in turn, causes an invasion of your own bubble) if it's not necessary? I especially hate this violation of code a) in the morning and b) when it's hot.
2. Don't sit on the aisle seat if no one is in the window seat. Let's not be greedy. I like personal space as much as everyone (except for violators of point 1 above), but there's no need to make people climb over you or uncomfortably ask you to move so they can use the vacant seat.
3. Don't sit so close you're touching me. There are some exceptions here, because obviously people don't have control over their size. However, some of you out there just sit too close. Like a Gilette Mach 20. I don't need that close of a shave, brosef. Scoot over.
4. Do wait for people to exit the bus before you get on. Calm down, eager beaver.
5. Do understand which seats lift up for wheelchairs. Not all of the seats in the front of the bus lift to make space for wheelchairs, so the whole front of the bus does not need to get up for one wheelchair. Conversely, if you are sitting in a wheelchair seat and you see the others on your bench get up, you should also get up. Don't make the bus driver ask you.
6. Do move to the back of the bus. If there are a lot of people getting on behind you, keep moving to the back. Don't stand right in the front with your backpack on, making everyone awkwardly squeeze by you to get to the open space in the back.
7. Don't take up a whole seat with your bags and whatnot. This one is closely related to number 2 above. You have a lap, you have under the seat; figure it out.
8. Don't clip your fingernails. It made the list for a reason, folks.
I feel like most of these rules should just come out of common courtesy, but I am blown away every day by the amount of people who don't know (or refuse to acknowledge) the code. Let's make the bus world a more pleasant place, everyone. For my sake.