Comment by bluGill

Only because most drivers are tailgating and so if someone touches the brakes everyone needs to do a panic stop just in case. If people maintained a safe following distance at all times there would be space to see the lights and determine that no action is needed (or more likely you just take your foot off the gas but don't flash your brakes thus not cascading).

Of course the above needs about 6 times as many lanes as any city has. When you realize those massive freeways in Houston are what Des Moines needs you start to see how badly cars scale in cities.