This whole "economic downturn" thing is tricky business. As I’ve mentioned, it may be helping boost transit ridership numbers as cash-strapped folks abandon their cars.

But those same cash-strapped folks are also buying less stuff (even if they are buying locally). Buying less stuff means less sales tax generated in Washington state. And because Seattle’s Metro bus service gets more than half of its revenue from a dedicated sales tax, this is not good news for Seattle’s primary mode of public transit.

To give it to you in (rather depressing) numerical form, King County administrators have said that Metro’s sales tax revenue losses over the two-year 2008-2009 period could total $100 million — that’s 800,000 to 1 million hours of bus service. (And that doesn’t count the time you’ll spend standing around at bus stops waiting for a ride.)

But hey, what about all that stimulus money that’s supposed to be directed Metro’s way?

Metro General Manager Kevin Desmond addresses that question (emphasis mine):

The four-county region of King, Pierce, Snohomish and Kitsap should get about $150 million for transit from the stimulus package, Desmond said. The best-case scenario, he said, is that Metro will get enough of that to fill its budget gaps through 2010.

But that would be a one-time infusion of cash, and in any case, the aid could be limited to capital spending, which won’t help the operating budget, he said.

"Bottom line, it does not solve our problem," he said.

Well, then, what about those increased fares (up 50 cents since early 2008)?

That helps, but has really only covered the effects of inflation and the fuel-price spike last summer, Desmond said. And he’s worried that if prices go up much more, riders will be chased off.

What to do? Metro has already cut operating costs, delayed construction projects, and borrowed from reserve funds. There aren’t many options left. But a new, local-option motor vehicle tax could provide hope, if legislators — and voters — approve.