well one reason is just traffic totals.
as you add cars to a road, the speed wouldn't drop linearly, more like exponentially.
it might be something to do with the people not familier with heavy traffic, but not totally, it also helps in large American cities, often the on and off ramps are a bit better than they are here (longer merge leanes, and the merge lane+ exit lane aren't the same.
there is also the theory of how traffic jams ovvur, one person brakes, and it causes all of traffic to slow down.
heres a good trafic simulator to play with, use the on ramp one, and play with traffic volumes to see what happens.
http://141.30.51.183/~treiber/MicroApplet/simFrame.html