Final answer:
The long diagonal streets in Washington, D.C., were designed in a manner similar to European grand boulevards, being influenced by historical urban planning like the Spanish Empire's Law of the Indies, where significant streets radiate from a central plaza.
Step-by-step explanation:
The long diagonal streets in Washington, D.C., were designed with a purpose that was influenced by a variety of urban planning concepts developed throughout history. These broad avenues, which cut across the traditional grid pattern, were designed similarly to the grand boulevards found in the urban planning of European cities, influenced by the Spanish Empire's Law of the Indies and the strategies utilized during earlier colonial times by empires like Rome, Spain, and Great Britain.
In the Americas, Spanish colonization led to the creation of cities with a central plaza surrounded by major streets, serving as a parade route and as an opulent promenade for the elite. This design philosophy was carried into the planning of Washington, D.C., where the wide, diagonal avenues connected various important focal points of the city, such as government buildings and memorials, creating both a functional and majestic urban landscape.