By Mugo Odigwe

CHICAGO (CBS) — In two days, more than 40,000 runners will descend on Grant Park for the 2019 Chicago Marathon, and more than 1 million people are expected to line the race route to watch.

With such huge crowds coming out for the second largest marathon in the world, it’s important for organizers to know exactly where the runners are at any time. Organizers call it crowd science.

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When you’re talking about more than a million people lining a course that spans 29 neighborhoods, there’s got to be a way to track the 40,000 runners along the way.

That’s where Chicago Marathon medical director George Chiampas comes in.

“We have tracking devices that the runners have. We also have personnel at every catch basin, so if a runner were to get on a bus because they’re dropping out, we could scan their bibs,” he said.

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Chiampas said crowd science is basically the ability to get the runners into the park, make sure they are screened, and get them onto the race course.

“All that information is fed into our forward command center, where we have data technology; screens that all of our resources and personnel are utilizing,” Chiampas said.

It’s important for security and medical reasons. For example, if a runner were to be waiting at an aid station, the forward command center would know where they are, and the quickest way to reach them, and provide timely information to loved ones.

“We’ve had runners that have gone into a medical tent, have had a serious injury, transported to a hospital. We’ve been able to call their family members even before that individual reached the hospital,” Chiampas said.

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The marathon kicks off Sunday at 7:30 a.m.