Which avenue?
At first glance, there is little rhyme or reason to which buses use which avenues in Downtown Seattle. In fact, there is some structure to these allocations, although there are a bunch of exceptions. Assigning rules does point to some situations where a swap or two could allow heavy bus users to commit it to memory.
For simplicity, let's only worry ourselves with outbound buses.
| Avenue | Rule | Buses that meet the rule but aren't here | Buses that don't meet the rule but are here |
| 1st | No buses | N/A | 125 |
| 2nd | Departing via I-5 South, I-90, or SR 509 | 630 | 37, 412*, 413*, 415*, 421*, 425*, 435* |
| 3rd | Leaving by neither I-5, I-90, SR 509, nor SR 520 | 37, 125 | 41, 304*, 355* |
| 4th | CT and ST buses that leave via I-5 North or SR 520 | 412, 413, 415, 421, 425, 435 | |
| 5th | Metro buses leaving via I-5 North or SR 520 | 41, 304, 355 | 630 |
* Although the bus is ultimately heading north, board it at a southbound stop.
There are presumably myriad reasons for deviations from the rules. The 630, for instance, starts from First Hill so going all the way to 2nd would take time. But the simplicity advantages of swapping, say, the 41 for the 37 and 125 are pretty clear.
A glance at the CT system map doesn't reveal any obvious rhyme or reason to the 2nd/4th split . As we have 5 years before all of those routes go away, perhaps a revision is not in the cards.
If I've missed something, please indicate that in the comments.