Why didn’t you bike to work today? It’s healthy, it cuts emissions, it’s a wonderful way to start your day.

Oh. Don’t have a bike, you say? Well of all the excuses for not riding, that would seem the most airtight. But soon, even that may not be viable – thanks to a movement called B-cycle.

B-cycle is a bike-sharing business that launches tomorrow (Earth Day) in Denver, dropping 500 bikes at various points around the city for anyone to use as they see fit (“fit” being the key word here). There are fees – both “membership” and “usage” – but for as cheap as 18 cents a day (with a yearlong membership and minimum usage), you can free yourself of your smog-spewing car.

The company is a collaboration between healthcare giant Humana, Trek Bicycles and ad/design agency Crispin Porter + Bogusky. Humana has been promoting bike sharing for years as a way to improve members’ health, and seeks with B-cycle to become the first nationwide bike-sharing system.

SmartBike, a predecessor to B-cycle, debuted in Washington, D.C., almost two years ago, but that hasn’t spread in the U.S., popular only in Spain, France, Italy, Norway and Sweden. And SmartBike certainly wasn’t the first such service in Europe – Copenhagen’s Bycyklen, rolled out in 1995, holds that distinction. Since then, a myriad of bike-sharing-type programs have taken hold, mostly in smaller iterations, across Europe and the U.S. Check out the International Bicycle Fund for a list of places you can share bikes.

Paul DeMaio, a bike-sharing consultant who’s Managing Member of Washington, D.C.-based MetroBike, feels the time is ripe for bike sharing to take off in the U.S. as it has in Europe.

“It's taken a bit longer for bike-sharing to catch on in the U.S. as bike facilities (i.e. bike lanes, cycle tracks, trails, and parking) are not as well developed here as in Western Europe,” DeMaio says. “Many U.S. cities have made great strides towards becoming bike-friendly during the last decade and having a network of safe places to ride in urban environments is a necessary precursor for bike-sharing. Bike-sharing will do well in the U.S. as it fills the niche of convenient, inexpensive, on-demand transit and allows folks to add activity into their lives.”

And B-cycle is banking on such assessments as it aims to bring the concept big time in Denver, where it’s rolling out 500 bikes at 50 stations across the city.

B-cycle bikes look (above) pretty cool: they feature three or eight speeds, lights and a basket that holds up to 30 pounds, and each is equipped with a tracking system that calculates mileage, calories burned and carbon offsets, with all info Web-accessible.

Denver isn’t the only Captivate market bound for bike-sharing bounty: Boston and Minneapolis will get their own systems this year, thanks to Montreal’s public bike program, BIXI.

Let’s be honest – if you’re commuting miles upon miles to work, you’re not gonna give up the car for a bike. But if you’re in the city, with a relatively short ride somewhere – why not? Studies have shown 60% of car pollution is created in the first few minutes of operation and 50% of car trips are less than two miles. Plus, you can burn 300 calories an hour, or more, riding a bike, according to the American Heart Association.

If you’re totally into the idea of bike sharing but your city isn’t yet, check out the B-cycle map, where you can request your town become part of B-cycle’s expansion.