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Why did Jane Jacobs believe that density and mixed uses can make city streets lively and safe?

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Aerial view of Paris showing broad, radiating boulevards lined with historic buildings, demonstrating Baron Haussmann's city planning design. — urban planning Georges-Eugène Haussmann's modernization plan transformed many areas of Paris through the addition of wider boulevards, better lighting and water sanitation, new parks, and improved rail transportation.

In her influential work, The Death and Life of Great American Cities, Jane Jacobs challenged the mid-20th-century modernist planning model, which favored large-scale clearance and isolated, single-use developments.1

Jacobs argued that high population density is not a detriment to urban life but a vital component of it. She believed that density, when paired with a diversity of uses and population groups, fosters a lively street life that makes cities safer and more attractive. By promoting "24-hour" districts where a variety of functions—such as residential, commercial, and social activities—occur throughout the day and night, she argued that cities create a natural, continuous presence of people. This constant activity serves as a form of informal surveillance and social cohesion, which she viewed as essential for maintaining safe, vibrant, and sustainable urban environments.1

Ultimately, Jacobs contended that the complex social fabric of a city is destroyed by the "inhuman orderliness" of modernist planning, whereas embracing diversity and density allows for organic, sustainable growth.1