What happened when Montag crossed the ten-lane highway?
Montag is forced to cross the highway as he flees the authorities and heads to the sanctuary of Faber's apartment. As he approaches the highway, however, he hears the sound of a fast beetle a few blocks away and realises that he is imminent danger, not only from the authorities and the Mechanical Hound but from the speed-loving drivers of his society.
Nevertheless, Montag begins his crossing. He focuses on the path ahead while keeping alert to the nearby beetle. As the speed of the car increases, Montag quickens his pace. Once the car reaches approximately 130 mph, Montag starts to run across the highway, but this causes him to drop one of his books on the ground. Instead of retrieving the book, Montag keeps running because the beetle is getting closer and closer. As the beetle is "almost on top of him," he stumbles and falls, and this fall saves his life:
An instant before reaching him the wild beetle cut and swerved out. It was gone. Montag lay flat, his head down.
At this point, Montag realises that the passengers of the car were not the authorities but were joy-riding teenagers. He realises how close he came to death and ponders if these teenagers were responsible for Clarisse's death.
What happened in this part is that Montag almost got killed. He is almost run over by some kids out joyriding.
We are told that this kind of thing happens a lot. We know that people are very violent in this society and that they are really pretty into killing other people. They are also into driving very fast. A combination of those things is what happens to Montag -- kids going fast in a car decide to try to kill him.
Montag thinks these guys were going around 130 mph. But he stumbles and that throws them off and he doesn't die.