Question: DuPont resident Charles Andrews wants to know why residents in much of Pierce, King and Snohomish counties pay the Sound Transit taxes, but Thurston County isn’t within the agency’s tax boundary.

“It is a no-brainer that commuters from Thurston County use the Sounder at DuPont Station and the Sounder at Tacoma and Puyallup,” Andrews wrote to Traffic Q&A. “Additionally, the commuters add to the congestion on Interstate 5 and do not pay their fair share that citizens of the other three counties pay. Thurston County should not be exempt from this tax. It can only improve their congestion problems, too.”

Answer: Under state law, only adjoining counties that each have populations of at least 400,000 can form a regional transit authority like Sound Transit, according to agency spokesman Geoff Patrick.

“The Sound Transit District could conceivably expand in the future,” he said. “Areas outside the district that would benefit from regional transit services can request annexation, with the requirement under state law that both the governing city or county council and voters living within the area in question must approve the annexation for it to take effect.”

Patrick noted that some people who live outside the tax district still contribute to the cause. About 80 percent of Sound Transit’s tax revenues come from the 0.4 percent sales tax dedicated to the agency (it soon will increase by 0.5 percent under a tax measure voters approved in November).

That means people who live outside the district help pay for regional transit services when they make a purchase inside the district that’s subject to the sales tax.

So here’s a question for Traffic Q&A readers: Should the Sound Transit boundary be extended to Lacey or even Olympia? To respond, go to this story on www.thenewstribune.com and post a comment.

Got a question about traffic congestion, construction, spending or other transportation issues? Send it to traffic@thenewstribune.com. Include your name, hometown and daytime telephone number. We’ll answer as many as we can. Find an archive of previous answers at www.thenewstribune.com/news/traffic/qa.