“Many systems require slack in order to work well. Old reel-to-reel tape recorders needed an extra bit of tape fed into the mechanism to ensure that the tape wouldn’t rip. Your coffee grinder won’t grind if you overstuff it. Roadways operate best below 70 percent capacity; traffic jams are caused by lack of slack. In principle, if a road is 85 percent full and everybody goes at the same speed, all cars can easily fit with some room between them. But if one driver speeds up just a bit and then needs to brake, those behind her must brake as well. Now they’ve slowed down too much, and, as it turns out, it’s easier to reduce a car’s speed than to increase it again. This small shock—someone lightly deviating from the right speed and then touching her brakes—has caused the traffic to slow substantially. A few more shocks, and traffic grinds to a halt. At 85 percent there is enough road but not enough slack to absorb the small shocks.”