Two-Lane Blacktop is literature on film! At first viewing, it may seem stylistic but plotless, as the casual observer without proper frame of reference will miss some subtle subplots.
The first subplot is the contrast of the genuine versus the wannabe, as revealed in the cars and their owners. There has always been a street-race rivalry between the the home-built hotrod and the checkbook-aquired factory musclecar (fellow gearheads will nod knowingly). This contrast extends to The Driver, who is earthy and real, and GTO, who is always playing a role. At first, GTO tries to stand toe-to-toe with The Driver, but he is eventually subjugated by the horsepower of the '55 and the mechanical know-how of Driver and Mechanic.
The second and more interesting subplot is the tension within The Driver, who feels more comfortable with machines than with people (perhaps machines are easier to control). Believe me, this type of personality exists - confirm with any gearhead or IT professional. His machine zen is interrupted by the hitchhiker, to whom he opens himself up (barely). The hicthhiker eventually leaves, and at the end of the movie he slides shut the window of the '55 Chevy, symbolically shutting out human emotion/interaction and returning to his mechanical world.
Watch this movie looking for these subplots, and you may have a whole new viewing experience.