At the end of the book, Montag is on the run, escaping through a maze of city streets in the dark. The hound is after him, the authorities are after him, and he is desperate to get away and find a safe haven. The hound has already stung his leg, so his movements are stiff and labored; at one point, he stumbles across the street, trying to move faster as his leg throbs with pain. It is then that a car full of teenagers careens into the street, sees Montag, and in the spirit of having a good time, they try to hit him with their car in some sort of bizarre dare.
This might seem like a very bizarre thing to do--what kids in their right minds would actually purposefully try to hit someone in by running them over in a car just for a dare? To answer this, we need to turn back to some of the things that Clarisse told Montag about teenagers and their habits and activities. She mentions that she doesn't fit in with kids her age, and describes some of their behaviors, and how they
"can't do anything but...bully people around...or go out in the cars and race on the streets, trying to see how close you can get to lampposts...I'm afraid of children my own age. They kill each other."
Kids in Montag's society are violent, and all about the thrill and dare of adventure. It is a way to blow off steam, to forget their miseries, and the cops, government, and parents all encourage the behavior and discourage any sort of punishment for it. So, a car full of teenagers, out for a thrill ride, almost kill Montag. He is only saved because he tripped and fell down; this puts him out of the way long enough for the car to miss him. I hope that those thoughts helped; good luck!