This year, city officials in Denver have decided that too many races are taking place, and they have put a one-year cap on new events for 2015. The Office of Special Events cited the strain races put on streets, parks, and traffic as reasons for the decision.

Last fall, the city council in Raleigh, North Carolina, made a similar move, voting to cap at 95 the number of 2015 races that are allowed to close streets.

These two cities are among the most recent in which a surge in the number of events has forced officials to change policies. While the moves sound radical, there are already 250 races held annually in Denver. In Raleigh, the figure has spiked from 45 in 2011 to the agreed-upon 95 in 2015. How many more races can these communities, with populations of 650,000 and 430,000, respectively, support, if they aren't already saturated?

The skyrocketing growth in events mirrors what is happening nationwide. In 2009, approximately 17,000 races occurred across the country. By 2013, the most recent year for which data is available, that number had climbed to 28,200, according to Running USA, a nonprofit group that provides industry analysis to help promote long-distance races.

Meanwhile, subjects in Running USA's 2015 National Runner Survey reported finishing an average of seven races in the last 12 months. At these rates, the race supply threatens to outpace the demand. “If we're not there yet in terms of a constriction [in the number of annual races], we're close,” says Rich Harshbarger, CEO of Running USA.

There were approximately 19 million race finishers in 2013, but because runners can choose from an increasing number of events, not all races are seeing an increase. Running USA reports that 42 of the 100 largest races in the country lost participants.

“No question, many markets in the U.S. are becoming oversaturated with events,” says Dan Cruz, spokesman for the Competitor Group, host of the popular Rock 'n' Roll series. Cruz cites greater Phoenix, where eight marathons are held, and Southern California, as two locales crowded with events. “There is a half marathon pretty much every weekend from San Diego to Santa Barbara,” Cruz says. For future growth, the Rock 'n' Roll series is focusing on expanding internationally. It's also adding 5Ks to its existing U.S. events the Saturday before its full and half marathons on Sundays—and challenging racers to compete both days.

Even the 4.78-mile Manchester Road Race in Connecticut, one of the country's first turkey trots, now contends with about 30 events of comparable length scheduled for Thanksgiving weekend in the state. James Balcome, Manchester's race director, says that's because his race has been successful. “The feedback is that some people get frustrated because they are in a 15,000-person race, and they can't race the way they want,” he says. “I think three or four races have started around us because of that.”

Competing for Entrants

The primary barrier to launching a race is securing an event permit. Because most cities collect event fees, it has been in their best interests to green-light races.

In the last year, however, communities have begun to rethink their approach. Grace Martinez, the community affairs liaison at the Denver Office of Special Events, acknowledges that, in her words, the 250 annual races “snuck up on us.” That averages out to five races a weekend, with most happening between May and October, leading to traffic jams and complaints from residents.

This year, the Office of Special Events will try to determine how many races the city can comfortably sustain. “We still get calls from people almost every day asking to put on a new race,” Ramirez says. “We have to say no to them because we can't absorb that.”

Alan Lind, the co-director of Denver's Platte River Half Marathon, decided to stop holding his 7-year-old Park to Park 10-miler in 2013. The former Labor Day weekend race topped out at about 1,100 finishers before dipping to 818 when Lind called it quits.

“Races keep drawing from the same group of people,” he says of the runner market in Colorado. At least 10 races are scheduled for Labor Day weekend in greater Denver, including five half marathons.

Race directors also note that the pool of willing volunteers has not kept up with the number of races, which means organizers have to scramble to find groups to man water stations, direct runners, and hand out refreshments.

A Growing Expense

When the cost of a 5K exceeds the expense of dinner and a movie, many runners start thinking twice about signing up. According to Running USA's 2015 National Runner Survey, runners spend an average of $25 to $30 for a 5K, $35 for a 10K, $70 to $80 for a half marathon, and $150 for a marathon. For a runner averaging a race every other month, that could easily add up to $400 or more per year. “I definitely have made adjustments because of the high cost of racing,” says R.L. Bynum, a runner from North Carolina who competed in seven marathons in 2014. “I rarely run any races shorter than a marathon, because I refuse to pay $30 or more for a shorter race, particularly a 5K.”

The races that are thriving in this crowded marketplace? The ones that provide an unmatched adventure, like the How to Increase Mileage Running, the Disney World races or, say, the Boots and Daisy Dukes 5K in Harshbarger's hometown of Wichita, Kansas.

As market forces work, event organizers hope races will improve everywhere. Racers will put their money where the quality is—and the marginal events will get left behind.