[peterwwillis]

The stopping distance of my car (a Lexus IS300) at 70mph is 171 feet. So I know I need over 14 car lengths in front of me for me not to hit anyone with brakes at full power.

Do I actually leave that much space in front of me every time I drive 70mph? Hell no. But the slower the cars beside me are going, the quicker i'm going to hit one if it pulls in front of me. If they were going 70mph too, I would only have to brake down to, say, 65mph, which would should only take a second. But if they're going 10mph, I now have to come to an almost complete stop to prevent from hitting them, which as you can see above is a lot of car lengths.