Economics 101. The interstate station managers know most people will pay more for not driving so far off-route to get to cheaper gas. It's not gouging, it's just good business savvy.
Of course, a smart manager knows he can game the system a bit. He'll drop his prices a few cents to get more business. Or, he can raise prices a few cents because his loyal customers will still shop there, at least for a while, and he won't lose many customers for a few days.
At some point, though, customers do notice and shift their purchasing pattern. And the other nearby stations see the new price and adjust accordingly. Soon everyone is charging roughly the same again.
This ability for mutually-antagonistic competitors to act in unison has always fascinated me. I can't decide whether it is "monopolistic competition" or "oligopoly".
http://academic.udayton.edu/PMIC/Chapters/chap14a.htm