Clarisse's uncle is arrested for driving too slowly on the highway. He was once caught driving at forty miles an hour; as a result, he was jailed for two days.
Clarisse tells Montag no one really notices the details in life, as no one is allowed to slow down long enough to observe anything. Basically, Clarisse and Montag live in a totalitarian world, where citizens aren't allowed individual, autonomous thoughts that aren't sanctioned by the government. To keep its citizens docile and subservient, the government creates draconian and arbitrary laws that permeate every aspect of life. Failure to comply is met with stringent punishment.
Clarisse maintains that people have to drive so fast that they can't make out what they're seeing. They must imagine a green blur stands in for grass, a pink blur stands in for a rose-garden, a white blur stands in for a house, and a brown blur designates a cow.