(this picture is kind of scary, because yes I am about to drive, clearly I'm half awake with hair in my face and I still don't have my driver's license...just my learner's permit)
When taking a long road trip with a friend or friends, I tend to start off with a lot of energy and a lot to talk about, after an hour or two the conversation dies out and then there's that silence. Some people fall asleep, I don't, mainly b/c I'm probably jacked up on a few Venti Americanos (which include 4+ shots of espresso). It's also important to keep the driver awake. This is when it is essential to put on some music. The radio could work, though at this point we are usually in another state and not familiar with the local airwaves.
Hence the importance of putting together great mixes for you road trip(s).