Seattle has traffic, and it's not going anywhere. Congestion wastes a tremendous amount of time. All those people stuck in a traffic jam loss precious minutes of their lives behind the wheel. The loss of time for a family member means less time with the family, or less time making money to support the family. So there are potentially two losses - economic or social.
Congestion creates more air pollution.
Those people aren't alone in traffic. They are also inside their idling cars, which are pouring out pollutants while everyone waits. In the summer, the problem worsens because drivers run their air conditioning, which causes the engines to work even harder and produce even more pollutants. If traffic were flowing more smoothly, much of this idling time would be eliminated, and air quality would improve. The fuel source of today's autos comes from fossil fuels, which are not renewable.