NEW YORK - The city is considering mandatory helmet use for all Citi Bike riders.

Mayor Bill de Blasio says there's also discussion about making cyclists obtain a license.

He acknowledged the health and environmental benefits to cycling, but said safety should come first.

"We have to think about what's going to be safe for people first, but also what's going to work. And I think the big questions are: Is it something we could actually enforce effectively? Would it encourage or discourage people from riding bikes? But I care first and foremost about safety, so we having those discussions right now," De Blasio said.

The helmet idea is already generating backlash from the advocacy group Transportation Alternatives.

In a statement they said, "Such practices are known deterrents to cycling, and fail to address the real challenges facing all New Yorkers: a culture and prioritization of cars over people."