Charlie travels through a literal tunnel three times during the novel. The first and third time, it's with his two BFFs, Sam and Patrick. The second time Charlie is by himself, and it feels a little different.
When Charlie is driving himself through the tunnel—on his way to a party to meet his friends—he spends a long time analyzing it. It's a really nice analysis, and he pretty much explains the symbolism of the tunnel better than we ever could—seriously, go check it out (4.13.84). But—no offense, Charlie—he takes all the magic and mystery out of it. Overanalyzing can do that.
When Charlie passes through the same tunnel with Sam and Patrick it's a little more magical. These are two of the few times he's actually living life. He's in the moment, not just on the sidelines watching and analyzing:
It was me standing up in that tunnel with the wind over my face. Not caring if I saw downtown. Not even thinking about it. Because I was standing in the tunnel. And I was really there. And that was enough to make me feel infinite. (Epilogue.21)
Tunnels can mean a bunch of different things, but one thing's for sure: they're a passage. So do they represent Charlie's passage from adolescence to adulthood? From wallflower to active teenager? From loner to loyal friend?
Or, hey, maybe all of the above… and then some.