There are a number of reasons why roads that were laid out prior to the automobile were often wide (and keep in mind that many, many roads were not wide... they were simply narrow animal or foot paths that grew into roads as populations grew and buildings were built around them... without planning, you tend to get city layouts like Boston rather than Manhattan above 14th street). On one hand, American cities arose at a time when the personal vehicle of choice was the horse drawn carriage. Unlike a motor vehicle, a horse drawn carriage could not back up or turn easily on a small radius. Unlike foot paths, a horse drawn carriage needed sufficient width to pass another carriage coming in the opposite direction, and having long, straight roads was preferred to the narrow, winding roads generally formed by foot traffic. So, many early American cities were built with sufficient space to allow for two way carriage traffic, parking on the side of the road without stopping traffic, and to allow horse drawn carriages to turn around relatively easily at slightly angled arcs. On top of that, many storefronts spilled into the streets, so the street widths in many planned cities were designed with the intent to facilitate foot traffic and storefronts on the sides of roads. Another factor contributing to the width of roads was the imperial approach to urban design that arose in Europe, particularly France and England. These designs were intended to flaunt the wealth and power of the aristocracy and rulers in such imperial nations, and included large scale, order, and uniformity. Hallmarks of this approach included long, wide boulevards, grand parks, gardens, and monuments, and radial road networks. The width was dictated both by the horse drawn carriage and the width needed to turn, but also by the excessive egos of the leaders of these nations, which led to demonstrations of excess. The roads were no exception. Washington DC's layout was designed in 1791 by French architect Pierre Charles L'Enfant, with similar goals in mind, as he intended to project a powerful nation through DC's design and layout. Finally, there was an attempt in some urban planning to avoid the traditional, narrow roadways, seemingly unorganized paths, alleyways and other perceived design flaws as a means to create spaces that were more open, clean, orderly, less crowded, and more desirable. John Randel Jr., the surveyor who laid our Manhattan's famous grid design with wide streets for the 1811 Commissioner's plan, said he wished to avoid "the frequent error of laying out short, narrow, and crooked streets, with alleys and courts, endangering extensive conflagrations, confined air, unclean streets, &c." Add to that the fact that in the early 1800s, germ theory had not caught on and there was a perception that confined spaces ("bad air") contributed to disease. So this was another motive in creating wider streets prior to motor vehicles. If this kind of history fascinates you, I recommend reading Streets and the Shaping of Towns and Cities by Michael Southworth and Eran Ben-Joseph.