NEXT summer, New York City plans to introduce its bike-share program, making two-wheeled travel possible for thousands more residents and tourists. Shared bikes, available at rental kiosks around the city, will have built-in safety features like a handlebar bell and front and rear lights that stay on all the time.

But the bikes won’t come with one basic piece of protective gear: a helmet.

That might seem strange to many New Yorkers, given the Bloomberg administration’s crusades against health and safety hazards, including tobacco smoke, salty foods and sugary drinks. Helmets, however, despite their ubiquity in the city’s advertisements to encourage bicycling, are not mandatory for adults in New York. And neither city officials nor cycling advocacy groups are proposing a law to require them.

Making helmets compulsory, they contend, could actually make cycling less safe. The more bikes on the street, their thinking goes, the safer bike-riding is; and helmet laws discourage people from joining in, because of either the cost and inconvenience of buying a helmet or fears of fines. City officials and some experts say helmet laws have hindered bike-share programs in other places.

“It’s a balancing act,” said Jon Orcutt, a policy director for the city’s Transportation Department who is overseeing the development of the rental program. “You don’t want to impose a regulation. You don’t want to be working at cross-purposes with a heavy-handed rule that depresses or reduces cycling.”