Dark
There are no cars on this road. Usually, the headlights of passing cars illuminate the front seat, and Sam can see where Dean's staring out the windshield. He can guess with almost perfect accuracy what his brother looks like right now, but the dark closes over them both until there's just their own headlights along the broken white line that divides the road.
It does more than that, though. It divides them, creates this palpable tension that has more to do with what just happened than the road itself. But with every few hundred white dashes Dean puts between them and the spot on the side of the road where he trusted Sam enough to tell him the truth, the void gapes wider.
Sam knows that Dean will blame himself, because that's what Dean does. He can't actually complain because he has a tendency to do so himself. This time, though, the blame isn't unfounded; Dean might trust Sam enough to tell him the truth about the three months (forty years) he was in Hell, but Sam still can't find it in himself to tell Dean about the three months he spent trying to get him back.
If there were other cars here, he might be able to see how white Dean's knuckles are on the wheel. It's unnecessary - he doesn't have to see to know that they are.