In Part Three of Fahrenheit 451, as Montag makes his escape across the highway, he learns an important lesson about the cruelty of young people. This occurs after a group of joyriding teenagers, driving at "five or six hundred miles" per hour, almost hit him with their car. Montag then realises how cruel young people can be: "For no reason at all in the world, they would have killed me."
In other words, Montag realises that young people in his society place a higher value on enjoying themselves (by driving fast along the highway) than on the lives of their fellow citizens. In their minds, Montag, as he walks across the highway, is nothing more than an obstacle on the road ahead and killing him is an act of little significance. Fortunately, however, Montag's life is saved, but only by sheer chance.
It is the callousness of these teenagers which makes Montag wonder if Clarisse was killed in a hit-and-run accident. This possibility infuriates Montag and makes Montag even more determined to escape the city and build a new life.
We learn about the cruelty of the young early on. Clarisse comments on how many teenagers die in car accidents, get into vicious fights, and kill themselves by overdosing. Toward the end, as Montag flees, the cruelty of society is illuminated. Because the authorities cannot catch him, they sacrifice an innocent man by posing him as Montag and setting the hound on him. He is attacked and dies on live television.