Race organizers have brought in a crowd science expert to help manage a field of more than 32,000 runners across the 42.195-kilometer course from Hopkinton to Copley Square, including stretches on narrow streets laid out during Colonial times.

“There are certain things that we can’t change — that we don’t want to change — because they make the Boston Marathon,” said Marcel Altenburg, a senior lecturer of crowd science at Manchester Metropolitan University in Britain. “Like, I’m a scientist, but I can’t be too science-y about the race. It should stay what it is because that’s what I love. That’s what the runners love.”

The Boston Marathon, the oldest annual marathon, was inspired by the race at the first modern Olympics in 1896. That event paid tribute to the route run by the messenger Pheidippides, who carried news of the Greek victory over the Persians in Marathon to Athens. After delivering the message, “Rejoice, we conquer!”, Pheidippides dropped dead.

Boston organizers want a better experience for their runners as the race has grown from 15 entrants in 1897 to as many as 38,000 for the 100th edition in 1996. The field has sat at about 30,000 since 2015.

That growth has pushed against the limits of New England roads and the needs of the eight Massachusetts cities and towns along the route, which want to reopen streets for commutes and business as soon as possible.

“It would be kind of great someday to be able to grow the race a little bit more,” race director Dave McGillivray said. “The problem with this race is that it’s about two things: time and space. We don’t have either. … So, we’re trying to be innovative.”

Altenburg, a former German army captain who runs ultramarathons, has worked with major races, other large sporting events, airports and exhibitions on crowd flow and safety. For Boston, which also draws hundreds of thousands of spectators, he used models and simulations to test how the race might play out under different conditions.

“We have simulated the Boston Marathon more than 100 times to run it once for real. That is the one that counts,” Altenburg said in a telephone interview. “They gave me, pretty much, all creative freedom to simulate more waves, simulate more runners and — within the existing time window — they allowed me to change pretty much anything for the betterment of the running experience.

“And then we checked every aid station, every mile, the finish, every important point, (asking): Is the result better for the runner? Is that something that we should explore further?”

The clearest change this year is at the start. Runners will leave in six waves, grouped by qualifying time, instead of four. Boston first used waves in 2011. The format helps spread out the field so runners do not have to walk after the gun on Main Street in Hopkinton, where the road narrows to just 39 feet wide.

Other changes are less visible. Organizers also adjusted bus unloading at the start, the placement of water and aid stations, and the finish line chutes, where runners receive medals, a mylar blanket or a banana, and medical treatment if needed.

“For an event that’s as old as ours, 130 years, it allowed us to be a startup all over again,” said Lauren Proshan, the chief of race operations and production for the Boston Athletic Association. “The change isn’t meant to be earth-shattering. It’s to be a smooth experience from start to finish,” she said. “It’s one of those things that you work really, really hard behind the scenes and hope that no one notices — a behind-the-curtain change that makes you feel as if you’re just floating and having a great day.”

Shorter porta potty lines would help too.

“What I loved about working with the BAA was how aware they are of what the Boston Marathon is. And they won’t change anything lightly,” Altenburg said. “So it was very detailed work from literally the moment the race last year ended to now. That we check every single option. That we really make sure that if we change something about this historic race, then we know what we’re doing.”

The BAA plans to review feedback over the next three years before making decisions on expansion or other changes.

“Fingers crossed, hope for the best, but we’ll get feedback from the participants,” McGillivray said. “And they’ll let us know whether or not it worked or not.”

Still, the course will not change, and organizers are not going to keep it open longer.

“I can talk. I’m a scientist. I just press a button and it’s going to be,” Altenburg said. “But the runners still have to do it.”