I am not sure how popular this idea is among UW students living in Tacoma, who might want to simply reverse the direction of the campus loop, and have it serve the U-District on the way to UW Station in the morning, and then have it serve the U-District last before heading down to Tacoma in the evening.
But, really, former 586 riders will not be the primary riders of route 591. With the rise of Amazon’s South Lake Union campus, among other multi-story employers in South Lake Union, there is plenty of demand for a route that not only makes use of the popular Seneca St. exit and drops off at Westlake Station, but also then heads into the South Lake Union business district. That demand exists right now.
Meanwhile, ridership on route 590 has ballooned from 2,139 daily weekday trips in 2012 to 3,011 daily weekday trips in 2013. (See page 77 of the 2015 Draft SIP.)
Let’s take an inventory of how many public bus routes from south of downtown Seattle are doing the SODO crawl, and how many are using the Seneca St. exit from I-5.
Exit from I-5 via Spokane St. viaduct:
King County Metro 101, 102, 150, 177, 178, 190
ST Express 590, 594, 595
Exit from I-5 via Seneca St.:
Metro 143, 157, 158, 159, 179, 192
ST Express 577, 578, 592
For Tacoma-to-Seattle commuters, there is no need to argue over whether the current route 590 or proposed route 591 path is better. There is already plenty of frequency in the peak direction to allow the trips to be split into 590s and 591s, plus the possibility of picking up a lot more riders with direct service to South Lake Union.
I realize I have totally avoided the topic of how route 591 would get to I-5 from South Lake Union and/or Westlake Station. Feel free to get creative in the comments.