The Benjamin Franklin Parkway can accommodate close to 300,000 pedestrians with ample breathing room, crowd-safety experts say.
But after months of preparation and shifting projections, how many people will actually show up to see Pope Francis this weekend? An even more critical question is how they will all stay safe.
The U.S. Secret Service says it is preparing for something close to the 300,000 number within the Parkway's fenced-in secure zones. Festival organizers project more than one million attendees for Sunday's Mass, including people who will congregate outside the secure zones.
A firm count likely will never happen. As much as people like to talk about crowd size, government agencies shy away from the politically fraught practice of counting people at mass gatherings. And in this case, a ban on aircraft over the papal events means there won't be aerial photos that could be used to estimate crowd size.
Safety specialists say precise numbers are less important than careful management to avoid disasters like the one Thursday near Mecca, Saudi Arabia, where more than 700 people were trampled to death during the annual hajj pilgrimage.
No one expects any such thing to happen here, but planners are well aware that a papal visit has special challenges.
"It's a once-in-a-lifetime opportunity to see him, to be close to him, maybe even to touch him if you're lucky enough, to be in his presence," said Paul Wertheimer, a Los Angeles-based crowd-safety consultant. "That highly emotional environment requires special planning."
Keeping big crowds safe requires plenty of entrances and exits, clear access for emergency responders, and use of barricades or evenly spaced attractions - such as Philadelphia's papal Jumbotrons - to discourage people from bunching up.
A crowd can change character in a hurry, said G. Keith Still, a professor of crowd science at Manchester Metropolitan University in England, who advised royal-wedding planners in 2011.
"You don't just consider the space where people might stand to watch the event," said Still. "It's how they move on and off the site."
Planning a safe event is something of an art.
"It's not a science, because we're human beings," said Wertheimer. "We're not mathematical equations."
Still said a density of up to five people per square meter - 10.76 square feet - is considered reasonably safe for gatherings where people are mostly standing still. But given that the crowd eventually will be on the move, he said, planners should shoot for no more than two people per square meter. That's roughly an area the size of a large coffee table for each visitor.
According to Inquirer calculations from aerial images, the area where pedestrians are likely to stand along the Parkway encompasses about 33 acres - including 12 acres in a ticketed zone on the roadway's northwest end, and 21.5 acres on the southeast end, ending with LOVE Park.
Using Still's guideline for a moving crowd, the 12-acre area could handle close to 100,000 people. Organizers have played it even safer, allotting 80,000 tickets for that zone.
The remaining 21.5 acres could handle more than 170,000 people using Still's standard.
Parkway events in the past may have drawn far more tightly packed crowds. In 2005, then-Mayor John F. Street said the Live 8 concert drew more than 600,000 people, but that's hard to verify.
This weekend, the U.S. Secret Service is not going to count people entering the unticketed area of the Parkway and cut off admissions at a predetermined level, Special Agent Robert Hoback said.
A safe crowd density depends on such factors as the age of the attendees, how many people are using wheelchairs or walkers, and whether people are using umbrellas, he said.
City and federal officials will monitor conditions inside the fenced-in areas. If things start looking too close for comfort, they will make a consensus judgment and close entrances, which will be equipped with metal detectors.
"When folks see that it's reached capacity, they will let our folks at the magnetometer checkpoints know that there's no room for anybody else," Hoback said.
Still and other experts stress that densities at big public events are not constant, and that a jam-up in one spot can ripple to surrounding areas. Trouble can arise if people start surging toward a central speaker or attraction, said Gil Fried, professor of sport management at the University of New Haven.
"You've got to have constant vigilance," Fried said. "Everyone wants to be in front."
It's too soon to know how packed the events will be, but transit ticket sales suggest that crowds could be lighter than expected.
The three regional rail services, PATCO, SEPTA, and NJ Transit, reported a combined total of 136,000 special papal passes sold in advance. That's about one-fourth the available number.
Customers with standard rail passes can still board PATCO trains this weekend, but SEPTA will admit only people with the special passes on Regional Rail trains.
Amtrak projects about 25,000 passengers using 30th Street Station between Saturday and Monday.
The Delaware Valley Regional Planning Commission projected this week that 34,000 private vehicles will be driving into Philadelphia and Camden, and 1,200 buses, each of which each can carry about 60 people, are registered to park at the South Philadelphia Sports Complex.
Hotels in the city reported that they have reservations for close to 10,000 rooms, or more than 90 percent of capacity in Center City and University City.
How many Philadelphia residents will attend is another unknown. Regular fare passes and tokens can be used on the city's public transportation lines, unlike the Regional Rail lines, SEPTA spokeswoman Jerri Williams said.
The DVRPC received an estimate of one million attendees Sunday from the city, revised down from an estimate of 1.5 million. But the mayor's office would not confirm the new figure.
"We're not offering any estimate," said mayoral spokesman Mark McDonald. "We know that there will be a lot of people."
The World Meeting of Families is still anticipating more than a million people at the Mass on Sunday, spokeswoman Meg Kane said. That estimate is based on attendance at the 2012 World Meeting of Families in Milan, Italy, and anticipated interest among American Catholics.
"We don't have an RSVP," Kane said, echoing a comment Mayor Nutter made Thursday. "We just don't know."
Some experts say it's possible that the figures are so variable because of the nature of the event.
"When people are talking about religious devotion, there's a tendency to be very generous in crowd size estimates," said Joel Best, a sociologist at the University of Delaware. "Nobody is very excited about hearing that the pope's crowd wasn't that big."
Whoever is right about the crowd size, the fact that the audience is largely Catholic and primarily English-speaking gives organizers two more safety tools, Still said.
Mass communication is relatively easy with loudspeakers or signs, unlike on the hajj, where dozens of languages are spoken. And, in a pinch, organizers could even broadcast religious music, encouraging visitors to break out in song - a custom not associated with Islam.
"Singing can help pacify a crowd," Still said. "It gives them something to do."
Inquirer staff writer Julia Terruso contributed to this article.