Though the number of men and women in the U.S. is nearly equal, more than 75 percent of all bicycle trips are taken by men.

According to 2009 census data, there were about 4 million more women than men living in the United States (155 million to 151 million), yet according to a study published in 2012 by researchers at Rutgers University only a quarter of all bicycle trips in the U.S. were made by women.

The numbers are similarly slanted in Canada and the United Kingdom. Yet countries such as Germany, Denmark and the Netherlands, who have more robust cycling infrastructure, see an almost 50-50 split in rider numbers.

So why then don't more women ride bikes in the United States? It's a vexing question, but according to several experts we spoke to, the answers are fairly straightforward, having to do with factors such as concern for personal safety and roles within the immediate family structure.

"Women are especially worried about having a safe place to ride," explains Kate Powlison, research analyst and communications coordinator for the advocacy organization Bikes Belong. "For instance the 1 percent of the population who will ride a bike anywhere, no matter the conditions, is overwhelmingly male, about 80 percent."

Other factors include the role of women in larger American society, says Carolyn Szczepanski, director of communications for the League of American Bicyclists.

"Generally speaking women are more responsible for childcare in the U.S. And they are more responsible for getting their kids from place to place," explains Szczepanski, specifically addressing the smaller number of women bike commuters. "That means they have to deal with more trip chaining where they go from one place to another to another running errands. In turn they have to consider how they are going to carry whatever shopping items they may have picked up, or transport their children."

Obviously that's often not possible on a bike.

Szczepanski adds that the stereotype of who a cyclist is, a hardcore racer dressed head to toe in Lycra, can be off-putting for some women. "But fortunately that is starting to change," she says. "Now we see more people getting interested in cycling across the board."

To keep this momentum rolling - and encourage more women to ride bikes - organizations such as Bikes Belong and the League of American Bicyclists are taking a proactive approach, backing programs such as the Green Lane Project, which helps build protected bike lanes in U.S. communities.

"These types of facilities are proven to attract more women to bicycling," explains Powlison, herself a passionate cyclist and racer. "The Green Lane Project launched in Austin, Chicago, Memphis, San Francisco, Portland, and Washington, D.C. We were inspired by bike-friendly countries like the Netherlands, and are bringing their lessons and designs over to the U.S."

These Green Lanes, some which are actually painted green, are areas protected from motor vehicles by curbs, planters, posts, or parked cars. The lanes are carefully engineered with particular attention to safety, efficiency, and ease of travel for all street users.

"Bike facilities such as green lanes and bike boulevards make bicycling feel safer and less stressful, and that is a big way to get more women - and more people in general - riding bikes," says Powlison. "In bike-friendly countries like the Netherlands and Denmark where bicycling is very safe and pleasant, you see an equal number of men and women riding, as well as the world's highest cycling levels overall."

Meanwhile, the League of American Bicyclists continues to refine its Women Bike Program, which has the overarching goal of getting "more women on bikes, riding on the streets of their communities and rising to leadership positions in the movement."

"One of the things I am really pushing is elevating stories about women in this movement and the efforts that are going on to get more women riding around the country," says Szczepanski. "We want to really accelerate the process that is going on already."

The ongoing CycloFemme campaign asks people to promise to inspire more woman to ride.

One such story Szczepanski points out is CycloFemme, an on-line resource that organizes an annual Global Women's Cycling Day that "unites riders, regardless of gender, age, ethnicity or bicycle preference to share in the joy of cycling." In its first year, there were 163 organized rides in 14 countries; in year No. 2 that number jumped to 221 rides in 31 countries.

CycloFemme also has an ongoing pledge campaign that asks people to sign a document stating that they promise to inspire one more woman to ride a bike.

"It's a great reminder that creating more cyclists is sometimes just as easy as getting your sister, mother, or neighbor to come along for a ride," explains CycloFemme founder Sarai Snyder. "Small steps can go a long way."