Or you encourage alternate transport, be it car pooling or mass transit. If driving a personal car is cheap, that's what people will do even if it's slow. People will listen to the radio, maybe read the paper, do their make-up, talk on the phone, and kill time in traffic in those and any number of other ways.
Throw a higher dollar number on the cost of commuting whether through gas prices or toll charges, and more people who have the option will start to look at the other alternatives - mass transit, or relocating closer to where they need to travel, or changing work/school locations tolocales nearer to their homes.
Right now, there is insufficient "pain" effect to force people to consider any of that, and as a result traffic volume and commuting distances continue to remain and rise at levels that are not sustainable in the long run.