Passengers using the rear door button to board in the rear must remember all coaches are not equal. Some buses require that you hold the button until the doors are opened at least half way. On other buses, the doors will slam shut and the chance to board may end right there.
The three-tap rule, or the three-second rule, generally means that the door will stay open long enough for you to climb the steps and tag in.
If you are standing alone in a Tenderloin bus shelter, come to the front door, especially if we stop and open it where you are standing. Wearing ear buds, or not seeing double-parked cars or construction barriers, may mean you will be passed up if you do not board at the front door.
Yes, back door boarding is allowed, but you have to understand we don’t want tailgaters with soiled clothing and bad body odor, talking to themselves, to enter behind you. You may find the green light is not on to push and board rear. This is a security issue! Please be aware of who is in the immediate area!
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The Dao of Doug: A Bus Driver's Perspective 