The Atlanta Journal-Constitution Peachtree Road Race still will be the world's largest 10K when it starts for the 39th year Friday.

With another large and long-standing 10K in Colorado adding participants every year, the question might be how long will the Peachtree be able to make that claim, and what race organizers might do if the distinction ever is in jeopardy.

Jason Getz/AJC (ENLARGE)

About 55,000 runners participated in last year's Peachtree Road Race.

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Atlanta's July 4 race capped its registration at 55,000 (the current number) in 1998 but has enjoyed the distinction of being the world's largest 10K since at least the 1980s, when the cap went from 25,000 in 1980 and rose to 40,000 10 years later.

Now, the 30-year-old Bolder Boulder 10K is right on Peachtree's heels. The annual Memorial Day race reported a record 54,040 registrants this year and eclipsed the 50,000 mark for the second consecutive year.

According a post-race report in the Daily Camera newspaper in Boulder, the race has seen a 14 percent increase in runners the past two years, and its director has no plans to cap the number.

"We never want to tell someone they can't participate, because that sounds a little too exclusive," race director Cliff Bosley told the newspaper.

Tracey Russell, executive director of the Atlanta Track Club, which organizes the Peachtree, said there are no plans to increase the field, but there have been some conversations about it. She joked that the subject was the topic of one of the first questions she was asked when she was hired 18 months ago.

It would never be as simple as just accepting more entrants, she said. Runner safety, crowd control and other logistics would also need to be addressed.

"The most important thing to us is that the experience for the participant isn't in jeopardy if we were to add any additional runners [just] to go after trying to maintain that title," she said last week. "I think it would be silly on our part to just want to be the world's largest without taking into consideration the impact that would have on the current participants and what that means for staging and set-up. There's a lot to consider.

"We haven't said, 'No, we're never going to raise it.' "

Boulder's race director Bosley has made similar comments, telling the Daily Camera: "We don't want to sacrifice quality for the sake of quantity."

Russell has visited the Boulder race the past three years and came away impressed. Boulder, a running and health-conscientious community, puts on a great race, she said, but its participant numbers benefit from a longer registration period, which includes race-day sign-ups.

The Peachtree, on the other hand, fills up in less than two weeks by runners who first register in March, four months before race day. This year, around 8,000 applications were sent back, even though the race had to move its finish line from Piedmont Park because of the drought.

There's no telling how big the field could be if registration was open-ended and there was no participation limit.

"While their numbers have grown, the nice thing that we still have here in Atlanta is the allure and the prestige of running the Peachtree, because we have a capacity and you have to get in," she said. "With the amount of demand, it keeps it exciting to get that Peachtree number."