When Pope Francis spoke about joy this weekend, he probably wasn't thinking about the ecstasy that comes from being able to stroll down the center of Walnut Street without a car at your back. Or the rapture of skateboarding the wrong way on Pine Street. Or the bliss of biking 20 abreast on Broad Street. Or the pure, giddy fun of playing touch football in front of the Convention Center on Arch Street.

The unprecedented shutdown of the five-square-mile heart of Philadelphia was driven by the need for security (or rather, the perceived need for security), but it inadvertently created the kind of car-free city that urbanists dare imagine only in their wildest dreams. The virtual absence of vehicles in the sprawling secure zone, from Girard to Lombard, was a revelation. Instead of locking us in, it turned out that the much-maligned traffic box liberated us from the long tyranny of the car.

Philadelphia has always claimed to be a walkable city, but this weekend we saw walkability redefined.

Tentative at first, people were still sticking to the sidewalks on Friday night when the Pope Fence went live. But soon they were rushing into the streets like toddlers too long strapped in their strollers.

By Saturday morning's Pope Ride, which brought 3,000 cyclists onto Center City's streets, they had begun to thrill to the illicit pleasure of blowing through red lights. I saw people sit down in the street to look at their phones, simply because they could.

It was as if we were celebrating a war we didn't know we were fighting. The streets were ours at last. Walnut Street was transformed into a Barcelona-worthy rambla, packed with strolling crowds whose only purpose was to be out and about among people.

At some point during that delirious, citywide block party, it began to dawn on us: Why can't Philadelphia be like this every day?

Why not, indeed?

While no one would advocate making the traffic box a permanent feature of the city, this carless weekend has opened our eyes to the possibilities of closing streets and limiting traffic. We've seen that closing Center City streets, far from paralyzing the town, can make it a more joyful place.

Let's say Philadelphia did close, or narrow, a few streets to create dedicated corridors for pedestrians and cyclists. That would eventually result in denser settlement patterns downtown. Having more people concentrated in Center City would make SEPTA's operations more efficient and help support improved service - something Pope Francis, a former Buenos Aires subway rider, would surely applaud.

The benefits would grow from there. As our streets became less congested, more people probably would feel safe commuting and doing errands by bike. That, in turn, would help small businesses, since we're more likely to poke our heads into a shop when we're not locked inside a car.

There will be skeptics, of course. Philadelphia has still not gotten over its ill-fated experiment with the Chestnut Street transitway, the car-free zone that lasted from 1976 until the early 2000s. But the city is a much different place today. Over the last decade, as the city has added 30,000 downtown residents, we've begun to question whether all the real estate we've ceded to cars for streets and parking is really necessary. We've made few cautious adjustments, reclaiming barren traffic islands and a few parking spaces for mini-parks, but we can do more.

Now we have real-time data to support shrinking the city's asphalt inventory even more. As Carolyn Auwaerter, an avid cyclist, observed after taking part in the Pope Ride, "we just got a free experiment in open streets." And not just us. On Sunday, Paris also shut down its historic center to car traffic to highlight the impact of automobile-generated carbon on the earth's climate.

One way Philadelphia could continue this weekend's experiment is by putting some of its streets on what traffic engineers call a "road diet." The canyon-wide expanses of Broad Street, JFK Boulevard, and Washington Avenue are obvious candidates. Imagine if we used some of their excess roadbed for wider sidewalks, protected bike lanes, and light-rail lines.

I just returned from a trip to Bilbao, a city in Spain known for its Frank Gehry-designed Guggenheim Museum. Although the museum's fame is deserved, it's not Bilbao's most remarkable accomplishment; Abandoibarra, the newly rebuilt waterfront promenade, is. Once a car-dominated boulevard, it has been refashioned with a wide pedestrian esplanade, a tram line, and a two-lane bike highway. The automobile area has been slimmed down to one travel lane and one parking lane.

After Saturday's Pope Ride, I ran into Jim Kenney, the Democratic mayoral candidate, who was walking down Pine Street (on the sidewalk) to the pope events. He told me he's interested in repeating this weekend's street closures, perhaps on August weekends. He said he would also consider creating permanent pedestrian and bike corridors in Center City.

After Mayor Nutter first revealed plans for the traffic box, it seemed that all we heard were the complaints about the miles people would have to walk. Obviously, it would have been better if SEPTA service wasn't stopped short of Center City. Yet, despite the grumbling, people did walk, often playing guitars and singing all the way.

It's no accident that Pope Francis chose a tiny black Fiat as his papal vehicle during his U.S. tour. It served as a symbol of both his concern with climate change and his disdain for material things. But we should also take it as evidence that cars are not his priority. We're more likely to care about the poor and homeless when we see them at close range, rather than through a car window.

On Friday evening, I saw a mother teaching her child how to ride a bike in the middle of Chestnut Street. "It's amazing," she told the girl. "There are no cars here, just people." Maybe someday that won't be such an amazing sight in Philadelphia.