The population situated along the United States-Mexico border is concentrated in cities which form bi-national adjacencies. On this geographical border, those adjacent cities are frequently thought of as "twin cities" or as a "bi-national metropolis." They have even introduced the term "trans-border metropolis" to describe the continuous bi-national urbanism formed by the neighbouring cities of each country, including San Diego and Tijuana. This continuous trans-border urbanism is considered a single urban unit whose integration is rooted in the relations and interactions between each pair of adjacent cities. This idea of the "trans-border metropolis" is naturally inclined to be impressionistic; it contains a theoretical fallacy that is null of any substantial indicators.
This essay focus on debating why the municipalities located in the northern Mexican border do not form a metropolis with their neighbouring cities situated in the United States.