The functional mix and urban proximity are highly relevant issues in the current debate about the reconfiguration and improvement of cities. The advantages associated with mix-use and proximity are multiple and contribute to creating more vibrant and sustainable cities. A city that combines its uses and functions is, by definition, healthier, as it reduces the carbon footprint by minimizing daily commuting and facilitating softer forms of mobility; it promotes a more efficient and responsible use of land, given that urban infrastructure is better utilized by combining activities in different time slots; it fosters social interaction among a greater variety of people, enriching the urban social fabric; and it fosters synergies between compatible uses, a fact that encourages the creation of new complex elements that can become solutions where unforeseen future demands can take root.

This research aims to contribute to the understanding of these phenomena by deciphering some of their key urban principles and exploring the factors and balances that underlie the combination of uses in the contemporary city. The research begins with an in-depth look at the metropolis of Barcelona, which is contrasted with a comparison of some similarly compact areas in three other Spanish metropolitan areas: Madrid, Málaga, and A Coruña.

The project—developed within the framework of the National Plan for Scientific, Technological, and Innovative Research, 2017-2020 of the Spanish Ministry, "R&D Projects - Research Challenges"—aims to dissect, measure, and parameterize urban functional and social mixing through the analysis of 25-hectare fragments in significant fabrics within compact metropolitan areas. Using open databases processed with GIS tools, a comprehensive and detailed analysis is conducted on the studied fragments, both qualitatively and quantitatively, resulting in parameters and mapping of urban mixture and proximity.

The innovative methodology has been implemented and tested in certain fragments of the Barcelona metropolitan area [MetBCN], a recognized example of compactness that treasures a strong tradition in urban design and planning practices. In a second phase, the same approach was contrasted with urban sections in other Spanish metropolitan areas. Madrid, Málaga, and A Coruña represent three characteristic sizes of Spanish urban agglomerations (5 million, 1 million, and 0.5 million inhabitants) and are presented in this research as complementary to the main case.

The final goal of the urban research is not only to provide a novel description of the selected urban fragments but also to reveal guidelines, evaluation tools, and reference indices that can be extrapolated to analyze other urban contexts and serve as inspiration for promoting and implementing combined urban uses in contemporary metropolises.