At first, it seemed as though nothing could go wrong. Dockless shared electric scooters began showing up on the world's streets in 2017, and the vanguard — techies, baristas, twentysomething daredevils — hopped on and rode, confident they were tilting against two looming threats, urban congestion and climate change. The future of scootering seemed so bright that the valuation of the largest manufacturer, Bird, went from $300 million in March, 2018, to $2 billion three months later, an astronomical leap, even by Silicon Valley standards.