According to one recent study, Seattle’s  traffic was the third worst in the United States (after Los Angeles and San Francisco); the average Seattleite spends over 50 hours per year stuck in traffic jams! If possible, try to stay on public transit, both for your own sanity and for the environment. By all means avoid the 3–7 p.m. weekday rush hour, when I-5 and the I-90 and Highway 520 floating bridges are often virtual parking lots.
One of the more difficult aspects of Seattle city driving isn't getting there, but getting back: Finding an I-5 entrance ramp can be frustrating, especially after you zigzag over and under the highway a few times. One sure cure: Head down 5th Avenue, because entrances to both north- and southbound traffic are along it. Another good, although very busy, way to approach the freeway is on Mercer Street; get in the right lanes for southbound, left lanes for northbound.
Driving around downtown Seattle is much more difficult than walking, and sometimes almost as expensive as flying to Portland, because at some point you've got to park the car. Parking downtown is difficult on weekdays—and costly: meters (if you can find one) are $1.50 per hour ($1.25 in South Lake Union), and private lots cost $9–16 a day. Be warned that the vigilant meter readers seem always to be waiting for your sticker to expire so they can stick you with a $20 ticket.
Seattle  has all the major national car rental companies, along with several smaller operators; surf their websites or call their 800 numbers for specifics. The big operators have desks near baggage claim at Sea-Tac Airport. It often pays to make an initial reservation online, using the 800 number, or with www.travelocity.com , www.expedia.com , or www.orbitz.com , and then do additional checking at the counters when you arrive. AAA or Costco members should be sure to also ask about any special discounts. The best rates are often from locations away from the airport such as Bellevue; sometimes the company will provide a shuttle van to its office. I've found that Thrifty Car Rental (800/847-4389, www.thrifty.com ) typically has some of the lowest rates and is a good starting point for price comparisons. For wheelchair-friendly van rentals, contact Wheelchair Getaways (425/788-7318 or 888/376-1500, www.wheelchairgetaways.com ).