On a Monday afternoon in November, I spotted a teenage couple on the Georgetown waterfront in Washington, D.C., sitting on a bench overlooking the Potomac River. Chris and Anna (who declined to give me their last names), were 19 and 18 years old respectively. They were scarfing down Chipotle burritos beside a pair of dockless bikes from one of the several private companies that have sprouted up across D.C. in the last few months.

The colorful new models—from Mobike, Spin, Ofo, LimeBike, and JUMP—…