Street widths are normally dictated by subdivision codes and local street design manuals. In Dallas County and the City of Dallas, for example, they specify a 50-foot right-of-way as the minimum for all residential streets.
In Maricopa County (the city of Phoenix, AZ), nearly two-thirds of residential streets are precisely 50 feet wide, as specified in local development standards and design guidelines.
In Cook County, the City of Chicago mandates a 66ft right-of-way—a historical anomaly that is based on the 66ft length of a surveyor’s chain.
Wide streets are not just a historical artefact. They are still mandated by subdivision codes today.