FOR a few blocks, Lin Dakang had Park Avenue to himself.

There were no cars besides those idling at red lights as his bicycle whizzed uptown at nearly 20 miles per hour. It was a moment of bliss on a hectic Saturday night.

But Mr. Lin had no time to savor it. A clear plastic takeout bag hung from his handlebars: a steaming order of chicken in garlic sauce and vegetable lo mein that would cool quickly in the evening air.

Seconds counted, and Mr. Lin glided through intersections against the light, pausing briefly to check the traffic before pushing on the pedals and engaging the small but powerful electric engine in his rear hub.

This first delivery of the dinner shift was more than a mile from the restaurant. Arriving, he surged off the bicycle, lashing it to a nearby pole with a heavy chain. A doorman stepped from the gleaming entrance of a prewar building on Park Avenue and pointed him to a side alley.