John Benjamin Woodard recently planted himself in front of New York by Gehry, a residential building in Lower Manhattan that's one of the tallest in the world, and scouted the delivery trucks. He watched where they parked — or, more often, double-parked — and how long they stayed. He watched as pedestrians and other vehicles struggled to maneuver around them. He watched as one never moved at all.

"I'll tell you, it's there right now," Woodard says. "It's there today. It's there every day. It owns this piece of street."

The goal of Woodard's reconnaissance mission, done as part of a graduate program in urban studies at Columbia University, was to find out just what the rise of e-commerce might mean for cities in the years to come. On the one hand, ordering most of your household staples from Amazon could decrease congestion and car use, since you're not taking anywhere near as many trips to the store as you used to. On the other, all those Amazon deliveries could just as easily be increasing congestion by putting more UPS trucks on the road, while at the same time freeing you up to travel for other reasons.

Consider it this way: people around the world seem to have a travel time budget of a little over an hour each day [PDF]. Before the rise of e-commerce, part of that time would have been spent in the service of purchasing goods. But if that budget remains fixed, then people today may simply buy something online, then hop in a car and go visit a friend across town. In that scenario, personal travel stays constant while commercial travel increases — a net gain of people and goods on the road.

"That was the basic premise: Is the e-commerce adding more to the traffic?" Woodard says. "Are we just generating more and more when we think we're eliminating?"

Woodard's case studies of the Gehry and three other residential apartments in Manhattan found the answer to those questions may very well be yes. Surveying the buildings for several hours at a time in the middle of the day, Woodard found that, on average, delivery trucks stayed parked for 21 minutes at a time, and two-thirds of them were double-parked. Extrapolating the data over a full day, in the case of the Gehry, that means delivery trucks alone occupy road space that's not a true parking space for seven full hours.

Then of course there's the safety element of this type of unintended congestion. Woodard found that delivery trucks very often interfered with the free flow of pedestrians. He collected data on this, too, but took pictures that make the case just as clearly:

Though Woodard's case studies were never supposed to paint an exhaustive portrait of the urban e-commerce problem, they do underscore how little is known about it. One study from way back in 2004 estimated that delivery trucks cause nearly a million hours of vehicle delay each year, but the stunning grown in online shopping since then (and the fact that companies like Amazon are reluctant to release their data) makes any precise estimate difficult. Many experts consider this process of moving freight that final mile to be one of the biggest forgotten problems facing modern cities.

At the core of the problem is street parking. In a dense urban area like Manhattan, where few buildings have the luxury of freight docks or loading zones, delivery trucks have little choice but to park at the curb. That leaves passenger vehicles and delivery trucks to duke it out for precious street-parking space, which in turn leads to double-parking, which in turn leads to general congestion.

Woodard fears that the simplest solution — removing street-parking — would be met with such an uproar that proposing it wouldn't do much good. "There's no magic bullet," he says. "But I think it does have to center around the parking issue. E-commerce is generating more traffic for vehicles. Where are those vehicles going to go?"

There are some other remedies in the works. Some cities (including New York) have experimented with late-night commercial deliveries, though that requires a lot of manpower and might create too much noise for residential buildings. Other cities, including London and Paris, have tried shifting deliveries to bicycles, though that has obvious limitations of its own. Little "freight villages" — places where trucks can more or less park for the day — might do the trick but would occupy valuable city real estate.

Woodard believes the best response may be some combination of priced parking and highly enforced on-street loading zones. For now, though, he simply wants cities to recognize the extent of the problem.

"It's definitely — from what I explored — it's an issue and it's going to be a growing issue as we move into the future," he says. "To get the word out that this is something people haven't thought about before, I think is key."