Road races are still popular in the U.S., but experienced a slight contraction in 2014 according to a new survey put out by Running USA.

The non-profit, which promotes the sport, found that in 2014, U.S. road races had 18.7 million finishers. That's down 1% from 2013. The number of events is down 1% too.

"The race market is over-saturated and new runners aren't necessarily doing 'traditional' road races," said Scott Bush, director of communications for Running USA. Instead, those runners are "focusing more on experimental and immersive races like mud runs and obstacle runs which don't necessarily count finishers," he said.

Out of that 10.7 million-finisher pie, runners 25 to 44 make up half. Bush says that's a "sweet spot because running is one of the easier activities for new professionals, busy professionals, new parents and growing families" in terms of participation. He adds: "It's just strapping on shoes and getting out the door, so if you only have twenty minutes to spare on working out, it's the easiest of options."

No matter what the distance, though, women still lead the way. In both 2013 and 2014, women made up 57 percent of U.S. road race finishers. That's a total of 10.7 million finishers in 2014. Women dominate most in the half marathon, making up 61 percent of the finisher field.

"Women participating in the sport has grown consistently over the past decade, so it's not a surprise that they represent the majority of runners now," he said.

The half marathon is the second most popular distance in the U.S with 2,046,000 finishers. The 5k is still the top distance with 8,300,000 finishers, making up 44 percent of all race finishers in U.S. road races last year.

The 10k and marathon make up spots three and four, respectively. The marathon has grown from 541,000 to 550,600 finishers from 2013 to 2014, but the number of 10k finishers has dropped by 5 percent in that same time frame.

Bush attributes this to over-saturation as well. "Many people are choosing to invest their limited races in half marathons or the 5k, which is easier to run and less training is needed," he said. There's also a lot of 5ks in the U.S., too: 15,100 events were held in the country last year, compared to just 3,700 10ks. There were only 2,500 half marathons, but they tend to be larger events than most 5ks and 10ks.

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