We need a tax on self-driving cars. Beneath eight states of the American Great Plains lies the Ogallala Aquifer, one of the largest bodies of groundwater on Earth. For centuries, extraction was constrained by the modest capacities of wind and hand power. At that rate, this 'fossil water' resource was effectively limitless. Farmers could draw as much as they wanted without ever running it down. worksinprogress.co/issue/esc… But in 1949 Colorado Farmer Frank Zybach invented centre-pivot irrigation. Combined with electricity and the centrifugal pump, farmers could now draw thousands of gallons per well per minute, enough to irrigate 40 acres at a time. Since then, the aquifer has gone down 10%, losing a Lake Erie's worth of water. It is down 50% in the dry parts, where it recharges just 0.02 inches per year. Without intervention, modern pumps will bring about the total end of irrigated farming in the arid parts of the Great Plains in 20-30 years. This is what I call the Ogallala Trap. Technological change can create a new tragedy of the commons. The telegraph enabled the destruction of the passenger pigeon; sonar, radar, and diesel enabled the industrial trawling that devastated the North Sea cod in a decade; chlorofluorocarbons came close to destroying the ozone layer. Self-driving cars are about to do the same thing to roads. When you can sleep, work, or drink with friends in a moving vehicle, you will take many more journeys by car. Roads, which are free at the point of use almost everywhere, will grind to a halt. People who have to go to the office or the hospital will be stuck sharing the road with people having beers, working remotely, and taking naps. There is a fix, but it depends on acting now, before autonomous vehicles go mainstream. Voters balk at being charged more for something they already depend on. The tax needs to come in as soon as possible. Waymos are already in dozens of cities and do millions of journeys per month. We have very little time left. If we want to save our roads from omnigridlock, we must introduce road pricing for autonomous vehicles.