During Clarisse's conversation with Montag, she mentions her uncle was once arrested for driving too slowly on the highway. He was driving at forty miles per hour (presumably to observe his surroundings) and ended up being jailed for two days.
Clarisse thinks this is funny and sad at the same time, but Montag is clearly uncomfortable about discussing such matters with her. Clarisse obviously disagrees with the rationale for why her uncle was arrested. She maintains that most drivers drive too quickly to appreciate specific details in their surroundings. Because they are not allowed to slow down to take in the scenery, these drivers can only guess at what they are passing on the highway.
This is an indictment of the society Clarisse and Montag live in. Clarisse and Montag live under an oppressive, totalitarian system which allows for very little personal agency and autonomy in daily living. Every citizen is expected to conform and adhere to strict, often arbitrary rules aimed at discouraging dissent or debate. As such, no one is allowed to drive slowly enough to observe the intricate details in their surroundings; little personal freedom is allowed in the society in which Montag and Clarisse live.
The government fears that, as citizens grow in awareness, they may begin to notice discrepancies between their lifestyles and those of the ruling elite's. Essentially, what these leaders fear is a populist uprising that may threaten their hold on absolute power.