When planning for new mobilities, it is important to be a little skeptical. Advocates often exaggerate the benefits and overlook significant costs. Here’s an example. Optimists predict that autonomous cars will reduce traffic congestion, crash risk, energy consumption and pollution emissions, but to achieve these benefits they require dedicated lanes for platooning (many vehicles driving close together at relatively high speeds). When should communities dedicate special lanes for the exclusive use of autonomous vehicles? How much should users pay for the privilege? How should this be enforced? Who will be liable if a high-speed platoon crashes, resulting in a multi-vehicle pile-up?