The marches raise equally compelling and so far mostly overlooked questions about public space and the role the downtown core will play in a city that is increasingly dense and increasingly Latino. In that sense, they qualify as the biggest architecture and urban-planning story to hit Southern California this year. One way to understand them, in fact, is not as an anomalous outburst of civic anger or energy but as a particularly clear message about how the relationship between Angelenos and the physical spaces of their city is changing as L.A. evolves, however fitfully, from a private metropolis to a collective one.

It didn't take much more than a glance at the most dramatic photographs of the protest on March 25, the so-called Gran Marcha, to sense that. The photo of protesters massing at the feet of City Hall, taken from a terrace at The Times' building by photographer Bob Chamberlin, already ranks among the definitive images of Los Angeles. It is also a universe away from the iconic 20th century views of the city, most of which show serene private residences of steel and glass.

In fact, you could begin a lecture on a changing L.A. by showing Chamberlin's shot alongside Julius Shulman's famous 1958 photograph of Case Study House No. 22 in the Hollywood Hills. The image by Shulman reflects a city whose appeal had to do almost entirely with private aspiration. In Chamberlin's, the public is so determined to come together — and to make a political point, no less — that it simply ignores the fact that downtown Los Angeles could hardly be less hospitable to a large group of marchers. There is no plaza there to receive them, just some steps, sloping lawn and a clump of trees at the base of City Hall.

The most surprising fact about the marches is that while they seemed, from afar, to suggest a massive, unified will, they were actually more indicative of how the city is changing on a piecemeal, neighborhood-by-neighborhood basis. Gridlock and changing demographics are combining to make the sidewalk, the front yard and the local boulevard, rather than the onramp or the fast lane, the building blocks for 21st century L.A. If you have spent any time walking on Broadway downtown, for example, you understand that the marches were just an exaggerated display of the sidewalk vitality that can be seen there every day.

L.A.'s shift from an automotive paradise to a collection of circumscribed neighborhoods is not new, nor is it limited to the parts of the city where new arrivals to this country are drawn. It has been visible for years, at least in nascent form, all over the region, including in Larchmont, Abbot Kinney and other high-end, retail-driven enclaves.

But it would be dishonest not to state the obvious, which is that immigrant neighborhoods, and particularly Latino ones, are driving the change, and that it has accelerated noticeably as Angelenos who arrived here in the 1980s and 1990s put down deeper roots. Indeed, a whole category of urban-planning theory, known as Latino Urbanism, has grown up to explore the ways that arrivals from Latin America have changed L.A. and other major cities over the last several decades. With California expected to gain 21 million new residents by 2050 — more than 85% of them Latino, according to one estimate — the focus of the new research is anything but academic.

"Latinos, like any other immigrant group, re-create what they know," James Rojas, a planner at L.A. County's Metropolitan Transportation Authority and founder of the Latino Urban Forum, has written. "In many parts of L.A. streets no longer feel like suburban America but have a look, feel and use of Latin American streets. From the numerous street vendors selling on Pico Union's narrow sidewalks to the murals of East Los Angeles, L.A. is changing from auto-oriented to pedestrian-oriented."

From “DIY city services” in Detroit, where private individuals agree to leave their porch lights on at night to compensate for a neglected public lighting system, to scavenger-recyclers in the dumps outside Liverpool, to the humble recycling of “homemade utility objects” by the people of Ivanovo (who engineer TV antennas out of household scraps), individual solutions to urban problems prove more effective than large-scale plans and initiatives. The practicalities of adjusting to diminished conditions provide a realistic counterpoint to grandiose visions of “redevelopment.”