It's not because people are driving slowly, it's because people are changing speeds erratically and unpredictably (and you can do this at a slow or fast driving pace). Most notably, these are the impatient, aggressive tailgaters who are erratically alternating between speeding up and braking hard out of frustration.

Here's a whole circle of relatively slow-moving vehicles. The traffic jam starts when someone brakes unpredictably, causing a chain reaction that is tough to bring back to equilibrium. The effects in this video would be identical whether all those cars in the circle were exceeding the speed limit or were driving far below the speed limit. In fact, we don't know the "speed limit" of the circle of cars, and it doesn't matter -- the jam starts when someone does something unpredictable, not when someone does something slowly.

This isn't a defense of slow drivers -- they do, in fact, suck ass. It's the erratic, unpredictable driving of the frustrated people behind them, trying to make the number on their dashboard meet or exceed the number on the speed limit signs, that are causing the rippling effects that lead to the phantom traffic jams behind them.

The slow driver may be the reason that the tailgaters drive erratically and unpredictably out of frustration, but it's the erratic tailgaters that are causing the traffic jams.

Of course this is an extremely unpopular viewpoint (even though it's fact, not opinion), but needs to be said. There is simply no logical reason to be the impatient, erratic tailgater when someone is driving slowly. They're driving a tad slow, so accept your annoyed fate and don't turn an aggravating situation into a dangerous one.