Denis Leary once joked, "You want to ride a bike in the city, move to China." The line, part of his first HBO special, got plenty of laughs as he talked about how annoying it was to share the road with bikes. "Are you fucking insane riding a bike around the middle of Manhattan in traffic?" he asked. "There's no speed limit in New York. Have you noticed that?"

But the joke has lost some of its humor as the death toll of urban cyclists continues to climb.

In the last three years, there has been a nearly 20 percent spike in the number of riders killed on city streets by motorists. This is seen as the result of both an increase in bikers, and also attributed to the belief that drivers aren't sharing the road.

According to an HBO Real Sports report, which airs this month, commuting to work by bike has increased 60 percent in the last decade. But while increased bike use has been good for the environment and for personal fitness, it has also created tension between riders, drivers, and even pedestrians. The result of this witch's brew has produced deadly consequences.

Crowded city streets in urban centers like New York and San Francisco have a chaotic atmosphere, with bikes and people and cars and trucks all moving at high speeds, sometimes just inches apart from one another. Despite some efforts to create bike lanes and make places like Manhattan more bike-friendly, an increase in biker deaths the last three years make some cyclists feel like society has a total disregard for them.

One-third of American workers commute 1-5 miles each way to the office, which is a short enough distance that the number of cyclists may continue to increase. But unless Americans change the way we think about bikes, more of them on the road will only make a dangerous situation even deadlier. The situation has officially boiled over and confusion seems to rule. A San Francisco biker was run over by a truck and killed in 2013, and the driver was never charged. Two pedestrians were struck and killed by bikers last year in New York's Central Park, and one of the bikers was never charged, according to HBO.

In June, another San Francisco cyclist was killed when he collided with a police car.

While bikers, pedestrians, and motorists in America are finding it a challenge to share city streets, Europe seems to have it down pat. HBO profiled the biking culture of Copenhagen, where half the city commutes by bike and casualties have been kept to a minimum. A cycling network throughout the city offers bikers protected lanes and stop lights, bridges and paths, and even bike parking at the mall.

According to HBO, the bike culture in Europe — Amsterdam is another city with lots and lots of bikes — grew out of people getting tired of sitting in traffic. Fifty years ago, Copenhagen was clogged with cars, so people found another way to get where they needed to go.

According to the Governors Highway Safety Association, which examines behavioral highway safety issues, 621 cyclists were killed in vehicular crashes in 2010. That number grew to 680 in 2011 and to 722 in 2012, for a total increase of 16 percent, while other motor vehicle deaths grew by only 1 percent.

The trend is troubling because until 2010 bike deaths had been on the decline. But more people are on bikes now than ever before and cities are struggling to maintain the peace between riders, drivers, and walkers.

For access to exclusive gear videos, celebrity interviews, and more, subscribe on YouTube!