After activists protesting the death of Philando Castile left the governor's mansion in St. Paul, Minn., on Saturday night, they marched through the city down Lexington Parkway and then onto the highway, across all eight lanes of traffic. There, some of them sat down, a provocative gesture of civil disobedience in the face of rushing commerce.

They were occupying a highway that, a half-century ago, was constructed at the expense of St. Paul's historically black community. Interstate 94, like urban highways throughout the country, was built by erasing what had been black homes, dispersing their residents, severing their neighborhoods and separating them from whites who would pass through at high speed.

That history lends highways a dual significance as activists in many cities rally against unequal treatment of blacks: As scenes of protest, they are part of the oppression — if also the most disruptive places to call attention to it.

"If you can find a way to jam up a highway — literally have the city have a heart attack, blocking an artery — it causes people to stand up and pay attention," said Nathan Connolly, a historian at Johns Hopkins University. "Highways still perform their historic role from a half-century ago. They help people move very easily across these elaborately segregated landscapes."

Block a highway, and you upend the economic life of a city, as well as the spatial logic that has long allowed people to pass through them without encountering their poverty or problems. Block a highway, and you command a lot more attention than would a rally outside a church or city hall — from traffic helicopters, immobile commuters, alarmed officials.

“We’re the home of Dr. Martin Luther King," anxious Atlanta Mayor Kasim Reed said on Saturday, acknowledging the city's legacy of protest but drawing a line at the interstate on-ramp. "The only thing I ask is that they not take the freeways. Dr. King would never take a freeway.”

That is not strictly accurate: King led the 1965 march that iconically occupied the full width of the Edmund Pettus Bridge in Selma, Ala. But as protests in Atlanta approached the high-speed artery that courses through the city's downtown, Reed understood that the stakes were much higher, both for the safety of the protesters and the functioning of the region.

That, however, is precisely the point.

"When people disrupt highways and streets, yes, it is about disrupting business as usual," said Charlene Carruthers, an activist in Chicago and the national director of Black Youth Project 100. "It’s also about giving a visual that folks are willing to put their bodies on the line to create the kind of world we want to live in."

A news helicopter in Oakland last Thursday night captured one such remarkable image on Interstate 880: a line of red taillights in one direction, white headlights in the other, as a narrow line of bodies blocked all lanes of the highway in between.

Later over the weekend in San Francisco, protesters tied up on-ramps onto the Bay Bridge, the city's lone direct connection to the East Bay. The protests in St. Paul blocked Interstate 94 for several hours, prompting riot charges against dozens of people. In Atlanta, protesters blocked the I-75/85 Downtown Connector.

The latest blockades, after a week in which graphic videos documented the police-involved deaths of Castile in Falcon Heights, Minn., and Alton Sterling in Baton Rouge, follow dozens of others over the past two years. Protesters in Chicago have blocked Lake Shore Drive. In New York, they've gnarled traffic on the Brooklyn Bridge. In Washington, they've targeted the 14th Street Bridge.

Researchers at the Rudin Center for Transportation at New York University, in a forthcoming study, counted more than 1,400 protests in nearly 300 U.S. and international cities related to the Black Lives Matter movement from November 2014 through May 2015. Half or more of the protests in that time in Saint Louis, Los Angeles, San Francisco and Oakland, Calif., wound up shutting down transportation infrastructure.

Black Lives Matters protesters gather in Kansas over last week's fatal police shootings of two black men. (Reuters)

"We systematically show that the political protest today is now almost totally focused on transportation systems, whether it’s a road, a bridge, in some cases a tunnel — rather than buildings," said Mitchell Moss, the director of the center and one of the authors of the study.

He draws a contrast with the occupations of schools, restaurants and administrative offices that commonly occurred during protests in the 1960s and 1970s, as an earlier generation rallied against segregated lunch counters or the Vietnam War.

Transportation, however, has long been central to the black civil rights movement, with the Selma march, the Freedom Rides, and Rosa Parks's appeal to equal rights on public buses. Fifty years ago this summer, the March Against Fear inspired by James Meredith walked 220 miles of Southern roads from Memphis to Jackson, Miss.

If anything is new, what's different today may be the occupation of urban interstates for the purpose of bringing them to a standstill. Protesters in Selma, Moss argues, wanted to use the Edmund Pettus Bridge — on their way to Montgomery — not block it.

Reed, who angered many activists with his comments in Atlanta, later defended them on Facebook by saying that King prepared for weeks and worked with Selma officials to ensure public safety, rather than flooding the bridge in a spontaneous and "dangerous" way.

To the extent that activists today are committed to a more urgent kind of disruption, planning ahead with police would defeat some of the purpose of bringing daily life to an abrupt halt, calling attention to the fundamental structures of inequality. And it's hard to imagine officials assenting ahead of time to closing an entire highway.

Highways also carry a particular resonance for the grievances today of black civil rights activists, given that many deadly encounters with police, such as Castile's, began with traffic stops (this patten has also prompted a new cry from transportation planners: "not in our name!").

Historically, the same thing that happened in St. Paul — where the black Rondo neighborhood was destroyed — happened in Minneapolis, and Baltimore, and Oakland, and Atlanta, and in Transportation Secretary Anthony Foxx's childhood home of Charlotte.

Planner Robert Moses used highways to clear slums through poor and minority neighborhoods in New York. Mayor Richard J. Daley used the new Dan Ryan Expressway in Chicago to wall off the old Irish white neighborhoods on the city's South Side from the black neighborhoods to the east where the city built blocks and blocks of high-rise public housing.

Black neighborhoods in the 1950s and 1960s had little political power to block these engineering behemoths. And cities that wanted to redevelop poor neighborhoods — another government goal of the same era — got more federal money by building highways through them than by appealing for "urban renewal" funds.

"If your goal was to clear slums," Connolly, the historian, said, "the best way to get bang for your buck was to use the highway as a slum clearance instrument."

The resulting highways were then meant to speed whites who'd moved to the suburbs back and forth to jobs and attractions downtown, leapfrogging minority communities along the way. As Connolly suggested, they still serve this function today. And often, highways that passed through black communities weren't planned with on- and off-ramps to them.

"They’re not designed for, nor do they serve, low-income communities who are actually already close to downtown," said Brown University historian Robert Self. "If you live in West Oakland, you don’t need a freeway to get to downtown Oakland."

This infrastructure that destroyed black communities then helped build white ones, in the form of far-flung bedroom communities that boomed once these roads made longer-distance commuting feasible. "Fremont exists before the freeway is built," Self said of the town 25 miles south of Oakland. "But once you build it, then Fremont becomes this massive possibility. Or San Mateo, or Redwood City."

Protesters in Washington who recognized this dynamic at the time objected to urban highways as "white men's roads through black men's homes."

Even without knowing this history, the consequences of it in cities are evident today, feeding the frustration of communities that have been segregated and separated from schools or parks or prompt ambulance access. The Dan Ryan Expressway in Chicago still divides neighborhoods and resources from one another, Carruthers laments. In Baltimore, black communities are still fighting for the resources their communities rely on — investments in public transit — as the state continues to prioritize highways that predominantly serve white communities.

In Charlotte and Miami, all that concrete still looms over minority neighborhoods.

"They’re massive, massive occupiers of space," Self said of highways. "When you’re flying through them in a car, you don’t think ‘this is actually an entire block or two or three of housing that had to go for this to be there.’ But there was a historical moment when that was housing."

People occupied these spaces long before they felt they had to occupy the roads we built on top of them.