On Dec. 13, the largest of the recent waves of protests against racist, aggressive and militarized policing tactics — spurred by the failure to indict the officers who killed Michael Brown and Eric Garner — took place across the United States.

In what was dubbed the Millions March, tens of thousands of protesters took to the streets, with New York City and Washington, D.C., being hubs of activity. (Some estimates claim as many as 50,000 protesters in New York and 25,000 in Washington.) For pundits and reporters, the reasons for and actions during these demonstrations are a bountiful source for analysis and pontificating. Yet there’s a character in all these stories that is always present but is not often granted the attention it deserves: the cities.

More precisely, I mean what I call the architecture of dissent, the ways in which the physical structures and flows of a city directly affect the ability for people to gather, coordinate and maintain visible social movements.

New York City has been the focal point for recurrent demonstrations over the last couple of weeks, with large, long marches, die-ins and rallies. This is not surprising, since New York is the most populous city in the country. But even more than that, the urban environment — dense, centralized, vertical, walkable — creates spaces that are conducive for these protests to pick up steam. The existence of public spaces, such as Union Square and Washington Square Park, function as easily accessible rallying points. The subway lines provide quick mobility throughout the city. The opportunities for targeted demonstrations are plenty, whether it’s shutting down an iconic thoroughfare like the Brooklyn Bridge or staging a die-in in Macy’s flagship store.

These features of the city, among others, allow a large and diverse crowd to come together and move as a collective. They allow for the demonstrations to be highly visible — drawing attention to their cause and amplifying their voices — by disrupting the normal flow of people and business. They allow for the march to become energized, able to sustain protests for long periods. Such actions can occur without these beneficial architectural features, but it is much more difficult when the urban environment is a barrier to social movement instead of a helpful platform.

For instance, consider Phoenix, which has a nearly opposite structure and flow from New York’s — sprawling, decentralized, horizontal, drivable — which gives rise to a vastly different architecture for dissent. I live in Phoenix, and even though it is the sixth-most-populous city in the nation, its bodily response to the nationwide protests has been relatively silent. And that’s not for a lack of people who are fired up with passion, concern and anger about the issues motivating these protests. Earlier this month, in fact, Phoenix experienced its own incident in which an unarmed black man, Rumain Brisbon, was killed by a white officer who thought he had a gun.

Phoenix did not see the large protests of other cities because it is not built for humans; it’s built for private vehicles. People just happen to live there. This character of the city closes off opportunities for civic action by isolating — and disempowering — people, leaving them with little or no opportunity for gathering in solidarity. Many people, especially those in marginalized communities, live in neighborhoods and towns that are effectively restricted from accessing large parts of the city. And the flows of activity in Phoenix are such that the downtown becomes a ghost town after 5:00 p.m. and on weekends, when people hop on the highway and retreat to their homes. So even if protesters managed to gather and march, they would be chanting at empty streets and closed buildings.

On Dec. 5, I participated in what was, to my knowledge, the largest, most energetic demonstration in Phoenix since the events of Ferguson, Missouri. The march was planned to take place during the monthly First Friday art festival, which attracts a large crowd in the late evening. The protest started with about 10 people and slowly absorbed more protesters — many of whom joined after arriving to peruse the art festival — peaking at about 150 people. Relative to Phoenix’s population, this is a paltry number of demonstrators, but for us in the moment, it was a triumph.

For three hours, until the festival ended, we marched up and down a half-mile strip, the only place even on a Friday where we would be visible and cause some kind of disruption. At one point the crowd decided to march over a mile through mostly deserted avenues to a police station. It was a symbolic gesture but also a somewhat disheartening one that demonstrated the effects of the city’s architecture of dissent: The urban environment restricted our presence to something of little material consequence.

These infrastructural aspects are, of course, not the only reasons for the successes and failures of protests in New York, Phoenix and other cities, but they are important ones that are easily ignored and taken for granted.