Downtown Seattle--as with most central districts in most medium-to-large cities--can't expand its network of roads. Everything that isn't a building, a park, or a parking lot is already devoted to road space or pedestrian space, and even if you wanted to do so you couldn't take away enough sidewalk to provide an extra lane anywhere. On top of that, any congestion relief that new lanes added would quickly be eaten up by the incredible demand for driving to business district destinations; induced demand and the Tragedy of the Commons at work. So even if extra car lanes magically appeared, traffic would be no better off in the long run. You'd just have more cars.
You simply can't win with cars in dense areas: once a terrible-but-just-barely-acceptable level of congestion is reached, equilibrium sets in and the number of cars entering the district doesn't change much. Logistically it can't increase (there's no more room), and it won't decrease because the hassle of dealing with that equilibrium level of traffic is still a fair trade for many people compared with the conveniences of driving. Look at any large city and you see the same thing. Terrible traffic is a staple of big cities because downtowns contain far more destinations than there are roads to facilitate driving to them, and that's not going to change. We've accepted that dense cities are valuable economically, socially, environmentally, and culturally, and we need to accept that as long as this is true traffic will accompany it. The best thing we can do is to provide alternatives to driving that are actually appealing to the average person.
These facts regarding driving downtown are important because of how they contrast with transit use downtown: by dedicating more space to buses, streetcars, and light rail we are actually able to improve the quality of transit trips, something we can't achieve for cars. With the recent advent of bus rapid transit (BRT) in Seattle, we've seen very clearly the price we pay for our decision to force buses to share lanes with cars. That price, of course, is consistent congestion and delays that effectively negates the entire purpose of BRT. This is what we're seeing with the D Line traveling from Ballard to Downtown, and plenty of other popular lines--particularly those that travel along permanently-congested corridors like Denny, or just about any area in downtown.
And back to cars, just as increasing car lanes wouldn't improve traffic downtown, decreasing them wouldn't worsen it. Instead, it would change the equilibrium level of cars entering and leaving the central business district. It would change the congestion vs. convenience equation for enough people that transit ridership to and from downtown would increase and single-occupant vehicle driving would decrease, but traffic congestion in general would remain relatively unchanged. Except for transit riders! There are real gains that we can make on behalf of those who choose not to commute by car, or can't afford to. They're the only real gains available to us, and we should seize them.
We have three choices. First is to stick with the status quo, which no one really likes. It's not good for anyone. It sucks to drive in Seattle, and it sucks to take the bus. Our second option is to take away what few lanes are currently dedicated to transit and turn them over to cars, and to expand road space for cars wherever we can, however little space there may be for it. This won't improve things for current car commuters because of the reasons discussed above, and will leave transit users worse off. Third is to take some lanes (some lanes--a tiny minority of the total road space) away from cars in order to vastly improve the reliability and speed of some of the most vital transit lines in the city. Some car drivers will be forced to find another way to get into downtown during rush hour, yes, but quite a few will also be happy to start taking the bus with it's newly-improved service. And those who continue to drive won't notice much difference, because traffic can only get so bad and we're already there.
There's plenty of complaining in the news about cars being picked on, but I don't hear much constructive criticism about how things can actually be improved, whether it be for cars, transit users, or both. The status quo is untenable, and we can't devote any more space to cars. What else can we do? I choose option three.