1) Is this an old tradition? How far back does it go?
Near as I can tell, a spate of highways and state roads started being named for important statesmen during the New Deal and the Works Progress Administration did all that highway construction. So, 1930s – maybe older in some places.
2) Is there a standard length of highway (2 miles, 10 miles, whatever) devoted to each honoree? If it’s not a standard length, does the length reflect the importance of the person so honored?
My observation is that it varies from region to region and yes, longer stretches of highway do tend to be named for people with more “importance” attached to their noteriety and/or reputation. Memorial Drive is a state highway named for the Confederate dead and stretches from downtown Atlanta all the way out to Stone Mountain, where I live. A resolution was presented for Rep. Cynthia McKinney to have a stretch named after her, it was voted on and passed, and so there’s a small stretch of road named for her on Memorial Drive – we’re talking a mile or two at the most. But maaa-aan, some (conservative) folks around here were/are pretty damned livid about that.
3) Is it more prestigious to get an interstate named for you than a US route, and more prestigous to get a US route than a state route?
It’s a pretty smoking hot honor to get any state or federally maintained infrastructure named after you, but generally speaking, in the American mindset of such things, the bigger the better, the higher up it went, the more prestigious. A U.S. Route named for you meant you got federal approval somewhere down the line.
4) Is this common in other states in the South? Elsewhere in the country? I know (at least part of) I-476 here in the Philly area is the Vietnam Veterans Highway, but I’m not aware of any other highways around here named for a specific person. (Rest stops on the Jersey Turnpike, yes, the Turnpike itself, no.) I think there’s a stretch of highway in Ohio named for golfer Jack Nickalas. Not sure about “common” but I’ve seen it in Tennessee, Georgia, Florida and South Carolina. Heck, there’s a stretch of 301 (or is it 601? I forget, they intersect downtown) in Orangeburg, SC named for the 7th Vice President of the United States, John C. Calhoun.
5) How do people get nominated? What’s the approval process like? Varies somewhat. On the state level here in Georgia, someone submits a resolution and the state senate votes on it. Here’s one for a parkway on a state road named for Trisha Yearwood.
6) If someone new came along who deserved honoring, and they were out of highways, would they just take a section away from someone else? We got a loOOOooot of highway in this country, twickster. Renaming attempts go on all the time (Cynthia McKinney, for example), but it’s pretty rare to see that happen. If someone disgraced themselves after something had been named for them, I can see the honor being stripped away before it being arbitrarily handed off to someone else.