United States Route 66, from Chicago to Santa Monica, California, was one of the most well-known and celebrated roads in the United States. But at some point the designation of that series of roads was removed. Why was this done? Weren't the roads still there? Why was this done to famous Route 66 when it wasn't done to other well-known US numbered routes like US 30 (Atlantic City, NJ to Astoria, OR) or US 1 (Maine to Key West)?
I'm a highway enthusiast. I'll direct you to this Smithsonian article: http://www.smithsonianmag.com/travel/endangered-site-historic-route-66-usa-52145829
Essentially, most of the US Numbered Routes were two lanes roads, widening only for traversing towns or going over mountains. The original routes planned out included primary routes starting with odd numbered routes going north-south and even numbered totes going east-west. For example, 41 goes North-South, and 60 goes East-West. Most of the early routes ran cross country. Other routes were added over the years, some for a limited purpose, like US 42, which runs from Lousiville, KY into Ohio to just past the Pennsylvana line for a distance of a few hundred miles. Most of the primary arterial routes had offspring routes, like 231,331 and 431, attached to them. The second two numbers reference the parent route. These highways often traveled through every single town along the way with few exceptions. This made travel by car very tedious on some occasions. Compare driving on a US Route to driving on an interstate, where cities that aren't considered major are largely avoided.
Route 66 was not a transcontinental route: it wound from Chicago to Los Angeles. It wasn't a direct route at all, and for long stretches traveled away from any towns of mention. As the interstate system grew, more direct routes were favored, so US 66's meandering, twisting route was abandoned.
In modern times, many US Routes remain two-lane roads but are very important to the local area. Some have been truncated or even upgraded to interstate levels, but some US Routes function for at least some portions as direct routes and are therefore more valuable. For instance, US 41 has yet to be totally replaced by an interstate, though I-69 may do that. The roads that provide safe, direct travel, have largely remained unaffected.
Edit: The route is still signed "Historic" in some areas, but it is no longer an official United States Numbered Highway.
It's still around, right? Just re-designated as Historic, but still number 66. http://roadtripusa.com/route-66/
The group that decides on such things, AASHTO, felt that US 66 was so thoroughly supplanted by interstates that the designation no longer served an important wayfinding purpose. In much of the West, the replacement interstates actually used the same alignment as old 66, so that the original highway became either the east- or westbound lanes, or was reduced to a frontage road. In retrospect, I think AASHTO gave too much importance to navigation and too little (or none) to "romance," and that they should have left the designation in place for historic reasons. Cross-country navigation by drivers is not the only rationale for numbered highways.