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.”

Bicycle helmets have long been an emotionally charged issue in New York. A recent commercial for Anderson Cooper’s new talk show that had him riding helmetless through the city’s streets led to a round of online condemnation. “New Yorkers are going to be stepping in bits of your grey matter if you don’t stop being a moron” and get a helmet, one reader commented on nymag.com.

The city strongly encourages wearing helmets but requires them only for riders under age 14 and professional delivery people. Those mandates are part of a law, signed by Mayor Michael R. Bloomberg in 2007, that requires employers to supply delivery cyclists with helmets and other safety gear, like lighting and bells.

About half of states have some bicycle-helmet laws, mostly for riders under 18, but few cities require helmets for adults. Seattle, which is considering a bike-share program, has required helmets for all riders since a law governing the unincorporated county and suburban cities was extended in 2003.

The Bicycle Helmet Safety Institute, based in Arlington, Va., has long supported helmet laws for all riders. Its director, Randy Swart, said the risk of brain injury, which unlike a broken bone or a cut may not heal, was reason enough to don a helmet for every bike trip.

Yet Mr. Swart said he understood the reluctance of other cycling advocates to insist on helmet laws. “Nobody wants to stir up the kind of opposition that you get when you introduce a mandatory anything,” he said.

While a few high-profile deaths have brought attention to cycling hazards in New York this year, the streets are safer than ever, said Seth Solomonow, a spokesman for the Transportation Department, as more people have taken to riding on marked bike lanes and paths.

The department has tracked the number of cyclists, injuries and deaths since 2000 to compile its Cycling Safety Indicator, which found that while overall ridership more than tripled, the numbers of deaths and serious injuries remained fairly steady.

The department has promoted helmet use, giving away 50,000 free helmets through fitting sessions since 2007, and is struggling to keep up with demand, Mr. Orcutt said. As the bike-share program goes into effect, the department will continue the offer, with plans to distribute discount vouchers toward the purchase of helmets at local bike shops.

But city officials have decided not to provide helmets directly with the rented bikes. Sharing helmets could pose a hygiene problem, they say, and helmets are effective only if custom-fitted to each wearer.

Requiring helmets can hamper efforts to promote cycling, some research shows. In Australia, provincial helmet laws discouraged many would-be riders from using bike-share programs in Melbourne and Brisbane, said John Pucher, a professor of planning and transportation at Rutgers University. “The issue is that forcing helmet use does discourage certain people from riding,” Dr. Pucher said.

Tel Aviv and Mexico City, hoping to spur greater ridership with bike sharing, recently repealed their helmet laws. Dr. Pucher and others also point to the low rates of injury and high rates of ridership in European countries, like the Netherlands, where few bicyclists wear helmets.

Mr. Swart of the Bicycle Helmet Safety Institute dismissed the comparisons with other countries, saying cycling was more ingrained in those nations. “I don’t think anybody has a good feel what real effect an adult helmet law has in a U.S. setting,” he said.

Transportation Alternatives, a bicycling and pedestrian advocacy group in New York City, encourages wearing helmets but has long opposed requiring them, favoring a broader approach.

“The only way to make it safer is safety in numbers, and quality infrastructure — like streets that reflect that cyclists are using them, dedicated safe space for riders, and education,” said Caroline Sampanaro, the group’s director of bicycle advocacy.

Some people even oppose wearing helmets at all. George Bliss, the owner of the West Village shop Hudson Urban Bikes and a longtime bicycling advocate, says helmets imply that cycling is dangerous, frightening away potential riders. Going bareheaded, he said, sends a message that city riding is safe.

Mr. Bliss recently wrote a piece in a neighborhood newspaper hailing the prospect that thousands of riders would hop on the new rental bikes “in their normal civilian clothing, in most cases without helmets.”

“This is going to change everything,” he wrote in the piece in The WestView News.

All the same, handlebars on the city rental bikes will bear a notice listing the cardinal rules of the road — including a reminder to wear a helmet.

Mayor Rahm Emanuel plans to spend nearly $150 million to make Chicago “the bike-friendliest city” in the United States. That challenge is considerable, given Chicago’s slow start compared with Portland, Ore., and other bike-centered cities, and Mr. Emanuel’s initial plan is drawing complaints about an inequitable distribution of the investment.

The Chicago Department of Transportation’s $18 million bike-share program is expected to begin next summer with 3,000 bicycles and 300 rental stations, to be located in areas with dense employment, residential development and retail. The Bloomingdale Trail, to be built in an unused two-and-a-half-mile rail line that runs from Wicker Park to Humboldt Park on the North Side, is expected to cost around $50 million over several years. The city planning commission recently approved designs for a $50 million flyover bridge at Navy Pier, the busiest section of the 15-mile lakefront trail.

But so far, the city’s lower-income areas include just one project: a protected bike lane on 18th Street in the 25th Ward, though more such lanes could be added in the spring as part of a four-year, $28 million construction plan. The alderman for the 25th Ward, Daniel Solis, is also the chairman of the City Council’s zoning committee, and he is traveling to Amsterdam this month at the expense of Bikes Belong, an advocacy group based in Boulder, Colo.

Oboi Reed, a lifelong Chatham resident and founder of the Pioneers Bicycle Club, said Mr. Emanuel is pursuing a good objective, but is on the wrong path.

“I definitely support getting more people on bikes because a lot of the common health problems African-Americans face are a result of not getting enough exercise,” Mr. Reed said. “My concern is that the lion’s share of the resources are going to go downtown and to the North Side — the South and West will only see a sprinkling.”

With the city facing a budget deficit of nearly $640 million and a double-digit unemployment rate, Mr. Emanuel may find it difficult to justify spending large amounts on bike facilities.

“It probably isn’t going to help many low-income and out-of-work folks,” said Mark Rank, a professor at Washington University in St. Louis who analyzes poverty and inequality. “You can’t spend all your money on a single priority, ignoring transportation or anything else. Given the situation in Chicago, this much spending seems a bit out of whack.”

From 2000 to 2009, the percentage of Chicagoans commuting by bike increased from about 0.5 percent to 1.1 percent. The growth is similar to that seen in other industrial cities like Milwaukee, Detroit and Oakland, Calif., but still lags behind Portland, which tops the United States with 6 percent commuting by bike.

Mr. Emanuel has set a goal of installing 100 miles of protected bike lanes — at a cost of $28 million — by the end of his term in 2015. Protected bike lanes are separated from car traffic by cones, curbs or other impediments. Chicago’s first protected bike lane opened in July on Kinzie Street. The second lane is to be installed this month, on Jackson Street, with another 20 to be built in the spring — all in locations chosen by the city.

Sam Schwartz Engineering, a firm based in New York that was hired by Chicago to design a 150- to 250-mile bike lane network, will hold a series of meetings over the next eight months to help determine the best locations for all future bike lanes.

“There’s been zero public outreach on where the bike lanes should go,” said Steven Vance, a former transportation department consultant on bike planning issues and co-founder ofGridChicago.com. Mr. Vance said he approved of the city’s efforts to increase ridership but questioned the first few bike lane locations.

The lack of outreach could be a concern, according to Alan Berube, research director of the Brookings Institution’s Metropolitan Policy Program. “If it’s done without public education and public input, there could be some real resistance,” he said.

Ben Gomberg, the Transportation Department’s bike program coordinator, said the city chooses wide streets that either see a lot of bike traffic or connect main arteries. To save money, the department also tries to piggyback on current roadway projects. The city has applied for state support and for federal clean-air financing that could total $50 million.

Mr. Berube said the bike initiatives could help in a city where the unemployment rate is more than 10 percent and nearly one in four residents live in poverty. “It can connect people to services, to work, and improve their health,” he said. “We need more jobs, but we need accessible jobs, too.”