I agree that most European (and many ancient) cities had no city planning and grew organically. Though, this is not what I'm talking about. Yet, there are still quibbles around this as many European cities were rebuilt many times. Sometimes this reconstruction was the result of war, sometimes it was sheer reconstruction out of Urban planning. "The city [Paris] is one of the most striking examples of rational urban planning, conducted in the middle of the nineteenth century during the “Second Empire” of Napoleon III to ease congestion in the dense network of medieval streets." [1]

Though, the reason why 1940 - 1970 is so important is because it is post-war and a lot was rebuilt in Europe while at the same time there was a lot of growth in American cities (the baby boom; federal investment in roads, etc..), and both European and American Urban growth and reconstruction were heavily influenced by "Modernism" [2][3]. "European engineers were sent in flocks to the US to learn from the environments in which these revolutionary ideas were playing out, returning with tabula rasa development plans to realise their own modernist dreams." [4]

Modernist Urban planning ideas started in the 1910's and on, but it wasn't until 1940 that there was the mass of opportunity for rebuilding and the funding to implement those ideas. "Modernist principles have shaped city-building since the beginning of the
twentieth century. Numerous authors draw a connection between modernist discourse
within planning practice and the rise of the Fordist paradigm (Irving 1993; Calthorpe
and Fulton 2001; Sandercock 1998). In following these principles, the North American
built environment has taken the form of low-density sprawl. This development pattern
is characterized by a dominance of single-family housing, a reliance on automobile
transportation and a strict separation of land uses." [5]

A key difference is that US civil engineers still are quite influenced by Modernism. For example, US traffic engineers continue to optimize for the throughput of vehicles on city streets rather than the throughput of people [6].

On the other hand, around the 1970s affluent European urban planners pushed back on "Le Corbesier" style planning and "Modernist planning fell into decline. "By the late 1960s and early 1970s, many planners felt that modernism's clean lines and lack of human scale sapped vitality from the community, blaming them for high crime rates and social problems.[59] ... Modernist planning fell into decline in the 1970s when the construction of cheap, uniform tower blocks ended in most countries" [7]

[1] https://www.bbc.com/future/article/20130722-revolution-in-pa...

[2] https://www.archdaily.com/604056/north-america-s-radiant-cit...

[3] [How a Controversial European Architect Shaped New York](https://www.smithsonianmag.com/smart-news/how-controversial-...)

[4] [Story of cities #36: how Copenhagen rejected 1960s modernist 'utopia'](https://www.theguardian.com/cities/2016/may/05/story-cities-...)

[5] http://www.etsav.upc.es/personals/iphs2004/pdf/003_p.pdf (page 3)

[6] https://www.bloomberg.com/news/articles/2019-09-30/a-swiss-l...

[7] https://en.wikipedia.org/wiki/History_of_urban_planning#Mode... - "Reaction against modernism"

-----------------------

Beyond the above, the more extended exerts below I believe make the same point I made. I would find it interesting where these are patently false and do not support the assertion I made earlier:

> "Modernism: In the 1920s, the ideas of modernism began to surface in urban planning. The influential modernist architect Le Corbusier presented his scheme for a "Contemporary City" for three million inhabitants (Ville Contemporaine) in 1922. The centrepiece of this plan was the group of sixty-story cruciform skyscrapers, steel-framed office buildings encased in huge curtain walls of glass. [....] He segregated pedestrian circulation paths from the roadways and glorified the automobile as a means of transportation. "

> "Reaction against modernism: By the late 1960s and early 1970s, many planners felt that modernism's clean lines and lack of human scale sapped vitality from the community, blaming them for high crime rates and social problems.[59]

> Modernist planning fell into decline in the 1970s when the construction of cheap, uniform tower blocks ended in most countries, such as Britain and France. Since then many have been demolished and replaced by other housing types. Rather than attempting to eliminate all disorder, planning now concentrates on individualism and diversity in society and the economy; this is the post-modernist era.[59]"

- https://en.wikipedia.org/wiki/History_of_urban_planning#Mode...

"Modernist principles have shaped city-building since the beginning of the
twentieth century. Numerous authors draw a connection between modernist discourse
within planning practice and the rise of the Fordist paradigm (Irving 1993; Calthorpe
and Fulton 2001; Sandercock 1998). In following these principles, the North American
built environment has taken the form of low-density sprawl. This development pattern
is characterized by a dominance of single-family housing, a reliance on automobile
transportation and a strict separation of land uses." (page 3)

"A significant individual embracing these values was the Swiss architect Le
Corbusier. Beginning his practice in the late ‘10s, he wanted to correct the ‘chaos’ of
the city and create an ideal order. His impact on modernist planning thought is
incalculable, and his ideas were widely applied in cities during the 1950s and ‘60s." (Page 4)

"Following a 1926 US Supreme Court decision to safeguard property values
from noxious land uses and neighbours, zoning became accepted as the principal
planning tool (Hall 1988). The result was the strict separation of work, home,
marketplace and social life. This move to create areas dedicated to specific purposes,
and to remove uses that conflict produced single-use central business districts,
uniform housing tracts, and dispersed shopping centres and recreational facilities." (Page 6)

"Transportation policy during the 1950s and ‘60s focused primarily on increasing
vehicle capacity on roads. Analytical tools considered highways and cars only, while
ignoring community design and public transit considerations. Instead of deciding
where development should go, engineers just looked at projected traffic trends and
designed infrastructure in an attempt to accommodate them" (Page 7)

"Inherent in the modernist project was a belief in the ‘tabula rasa.’ As a result,
enormous areas were cleared with completely new environments inserted. Again, Le
Corbusier led the drive with his unrealized 1925 proposal to demolish historic Paris
north of the River Seine (except selected monuments that would be moved), and to
replace it with eighteen 700-foot towers (Moe and Wilkie 1997)" (page 12)

http://www.etsav.upc.es/personals/iphs2004/pdf/003_p.pdf