Why does traffic slow down during rush hour when the speed limit is the same as non rush hour?

• I have yet to understand why more cars going in the same direction @ the posted limit slow traffic down if the speed is the same the rest of the time.

• Answer:

  There are 2 main causes of this, both of which are reflective of the "human input" component. 1) Fractions of seconds differences in speed/time. 2) Human "error" (see: stupidity) 1) As we are not robots or machines, it can be difficult for us to keep a consistently balanced distance between ourselves and the car in front of us. While sudden and significant changes in our speed (expressed in temporal terms) is easier to notice, the subtle changes can have a significant effect as well. This is essentially indicating that if the lead driver is pacing at 70 mph, the next driver will have trouble maintaining that same 70 mph pace at a legal distance behind them. *Now* if both drivers had no other variables to consider, they could both use their "Cruise Control" to maintain 70 mph. Again, slight differences might abound from the exact calculations of this "Cruise Control" but let's move on. Since there are other variables and most drivers don't bother with cruise control in heavier traffic, their speed will fluctuate. So, if the *average* speed of the lead driver is now 70 mph, the best *likely* average speed of the trailling driver will be any speed less than 70 mph. Even if it is 69.9 mph, the difference is significant. It is significant because of the drivers behind them. Subtract 1 mph from the posted limit (55 mph in Illinois) for the number of drivers behind the lead and you can easily see why you're driving 10 mph on the highway! Even if you were to only subtract 0.1 mph for every driver, it isn't hard to reach this conclusion in rush hour traffic. 2) That being said, you can't really "blame" any person for not being able to maintain the *exact* same speed as the car in front of them at a legally permissable distance between them. But you can blame them for ****ing up the system, nonetheless. If you can get out ahead of the main congestion, you will typically find *at least* one "idiot" who refuses to cede to the natural flow of traffic. In Illinois, it is illegal to drive in the left-most lane unless you are passing. So the idiot that sits out there at the start of congested traffic doesn't seem to realize that THEY are *in fact* the MAJOR cause of traffic congestion. If there were only one of these people on the road at any time, his/her impact might be lessened somewhat. However, there is always more than one on the road at high traffic times. Still, if you come back to the "chaos/butterfly" effect on all traffic to the rear, even one person in this system can do a fair amount of damage.