Dawn imagines that this is a family vacation like the kind she's never really had. Buffy and Spike talk quietly in the front seat, and she pretends that it's about scenic overlooks and greasy-spoon diners instead of Slayer stuff and hellgods. Buffy's sitting pretty close to Spike, who's got his arm on the back of the long bench seat. It looks cozy. Dawn relaxes against the window.
When she closes her eyes, the windows aren't covered in black paint, and they're driving towards something fun instead of away from something awful. This is what safe feels like.