As traffic gets denser, (less space between vehicles), the road's capacity declines. Capacity is cars per minute that pass a fixed point. Traffic gets denser when cars are arriving faster than they are leaving the jam. The jam itself reduces the arrival rate of the road ahead and its capacity increase, thus clearing the jam. No impediment is necessary.
you missed it?! O_O There was parade with animals, clowns, and everything!
Right? I've always wondered the same thing, once you get passed a certain point, it just opens up like nothing happened.