Someone once said that if horses are in a movie, then it's a Western. I'm not sure I would go that far. As far as road movies, I suppose you could come up with a number of different criteria, but let's keep things simple. After watching a great road movie you don't talk about the plot, or the story (there often isn't one), you were just entertained by the cars, motorcycles etc. Admittedly, my list is skewed towards 1970's stark and existential road movies, because that's what I remember watching.
The List
In no particular order:
1. Vanishing Pont (1971): Kowalski (Barry Newman) has to drive a white 1970 Dodge Challenger from Denver to San Francisco in 16 hours. Why? Who knows? He then picks up some speed at a biker bar and starts tearing across the desert. A blind black disc jockey called Super Soul (Cleavon Little) sympathizes with Kowalski and provides updates on law enforcement activity. Naturally Kowalski runs in to an old codger who catches snakes for Pentecostals...and a naked hippie chick riding a motorcycle. Oh, and Mountain's Mississippi Queen is in it. Very cool.
2. Two Lane Blacktop (1971): GTO (Warren Oates...does anyone play a better burned out old man than him? Maybe Ben Johnson. RIP WO.) is tearing around SOCAL in a 1970 Pontiac GTO. He runs in to Driver (James Taylor) and Mechanic (Dennis Wilson..how cool is that?) tearing around in a highly modified 1955 Chevrolet 210 sedan. Harry Dean Stanton is also in it of course. They enter a drag race competition. There's a girl. There's hitchhikers. They agree to race to Washington, D.C for "pinks". What is the plot? None that I can think of...but great.
3. Dirty Mary, Crazy Larry (1974): I remember seeing this at the drive-in. Larry (Peter Fonda) and Deke (Adam Rourke) are two NASCAR wannabe's who extort a local supermarket manager for 150,000. While driving in their modified 1966 Chevrolet Impala, they come upon Tracy (Susan George) who joins them. Local cop Vic Morrow spots the Chevy so our trio trade it for a 1969 Dodge Charger 44o R/T. Soon they are tearing around the back roads while being pursued by a fleet of Dodge Polara patrol cars (what..no Plymouth Fury's available?). Of course the Polara's crash and burn trying to keep up with the Charger..even Bubba's modified "interceptor". How cool.
4. Electra Glide in Blue (1973): John Wintergreen (Robert Blake) is a motorcycle cop who patrols the Arizona highways with his partner Zipper (Billy "Green" Bush). There's hippies. A murder. Hippie hating Detective Have Poole (Mitchell Ryan). John shoots Zipper because he goes crazy. John then stops a van full of hippies on the highway and they blast him with a shotgun. The end. What does it mean? Who cares? Hippie revenge for "Easy Rider" maybe?
5. Easy Rider (1969): Wyatt (Peter Fonda) and Billy (Dennis Hopper) make a big score on a cocaine deal so naturally they head from LA to New Orleans to party during Mardi Gras. Along the way they stop at a hippie commune then get thrown in jail in New Mexico where they run in to George Hanson (Jack Nicholson), an alcoholic lawyer who joins them. They stop at a diner in Louisiana where the local boys don't take to kindly to our trio. That night while sleeping they're ambushed and George is beaten to death. In New Orleans they drop acid and have a "bad trip". The next day on the highway a couple of local yokels attempt to scare them with a shotgun blast but cause Billy to crash. The pickup turns around and next we see a burning motorcycle. The end. Great soundtrack (Jimi Hendrix, Steppenwolf, The Byrds).
Theme?
I don't know...maybe old timers back then really hated long hair?