Why jams occur? The simple answer…

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:

  1. Why is train travel more expensive than driving?
  2. Why do people drive to the shop down the road, rather than walk or cycle?
  3. Why do people in the US, and increasingly in developing Asia, demand more roads, and not better public transport?
  4. Why people use their garages for storing junk, and the street for parking their cars?
  5. … and more.