In that situation, traffic is going to go slow, no matter what.
That isn't the type of traffic jam I'm talking about.
There is also another type of traffic back up. The kind that happens in moderate traffic. Everyone slows down, sometimes even to a complete stop, and then a few hundred feet later, you are moving again at 50, 60, 70mph, as if nothing happened.
Sometimes this happens because there is the aftermath of a crash in the shoulder, or even across the divider on the opposite shoulder of the oncoming lane, and all the drivers feel it is very important for them to take a good look at it, because humans are just like that. Other times its because someone is getting a traffic ticket, and, even though the cop is clearly busy at the moment, people imagine they are more likely to be caught speeding if the can see a police car.
But most often, these slow downs happen for no apparent reason at all. You get to the front of it, and cars are accelerating just as suddenly as they slowed down.
Sometimes traffic pulses like this, fast - slow - fast - slow - fast - slow for miles. In some places, not quite as dense as in the first example above, the daily commute does this pulse jam every single day.