We had to know more, so we followed up with the caller, and she put us in touch with her husband, Gary Levitt, who now works in Boca Raton, Florida.

In 1971, Levitt was a principal with now-defunct Philly ad agency Sonder, Levitt, and Sagorsky, and he had recently returned from a trip to Copenhagen with his wife Barbara. While there, he became inspired by the city’s famous walking streets and car-free plazas like Strøget, and he wanted to bring the concept back to Philly.

The agency, whose office overlooked Walnut Street, came up with the concept of the Walk on Walnut Street—a car-free promenade on Walnut from Broad Street to 20th on a Wednesday evening after work. They selected a date, May 21, 1971, and in the weeks leading up to the event, they published a weekly newsletter to get the word out, which they carried around to Walnut Street businesses every Friday.

“We would go around encouraging individual retailers on Walnut to come up with something interesting to do on the sidewalk from 6-9pm, about three hours, during the week. They would do it, and then we would write about it in the newsletter.”

The firm also did a poster campaign to raise awareness over the two months leading up to the event. The poster featured a black and white image of “a diverse bunch of Center City folks” in the middle of Walnut Street, which the group got a permit to block off for the photo. It said “Take a Walk on Walnut Street” and most of the retailers put it in their windows, Levitt recalled.

Though skittish retailers were worried sick the day before the event that nobody would show up, he said, Walk on Walnut Street organizers lucked out with great weather, and thousands of people opted to hang around after work.

Levitt recalled watching from the firm’s office windows as “tens of thousands” of people began pouring onto Walnut and the side streets.

“We weren’t unsophisticated,” he said, “We certainly wanted to stack the odds in our favor, so we scheduled it for a time when people were leaving their offices. The street just filled up almost magically, within 15 to 20 minutes.”

The firm recruited some bands to perform for free on the streets (the Electric Factory was one of their advertising clients), restaurants extended their seating out on to the sidewalk, and there was some other light programming here and there. The main attraction though was the simple opportunity to stroll Walnut Street unencumbered by car traffic.