Jams occur because a scarce resource, the road, is underpriced, so more people drive than it can accommodate.
The Economist: Why driverless cars may mean jams tomorrow
“Underpriced roads” is also the answer to:
- Why is train travel more expensive than driving?
- Why do people drive to the shop down the road, rather than walk or cycle?
- Why do people in the US, and increasingly in developing Asia, demand more roads, and not better public transport?
- Why people use their garages for storing junk, and the street for parking their cars?
- … and more.