If roads are clearer, then people tend to move farther from work, which increases the number of cars on the road at any given time. Then the roads clog up again. Increased highway construction has been enabling people to move farther and farther out into exurbs (which people think will be nice places to live, and cheaper, but that turn out create rather shabby quality of life compared to walkable cities).
"(which people think will be nice places to live, and cheaper, but that turn out create rather shabby quality of life compared to walkable cities)."
"Shabby quality of life" by whose standards? The people who actually live there? I think not.
If life in the suburbs actually sucked, people simply wouldn't stay there. It's not like they're given a one-way exit visa when they leave the city. They could move back any time they wanted. But, as a group, they never do. The reason they don't is because they find the amenities found in the suburbs (in specific, safety, cleanliness, room for their possessions, privacy, and better schools) preferable to those found in the cities. "Preferable" here means preferable by their standards, which are not the same as those of a single 22 year old programmer, or a single 30-something art connoisseur, or...
