This one comes to Streetsblog via the Sightline Institute’s Daily Score, a blog covering environmental issues in the Pacific Northwest.

Why do traffic jams materialize for no apparent reason? In an effort to answer that question, here’s a surprisingly simple experiment cooked up in Japan by the University of Nagoya’s "Mathematical Society of Traffic Flow:" 

If you are the kind of transportation geek who finds this sort of thing fascinating then you’ll also really love this web-based traffic simulator out of Germany. But "prepare to lose your afternoon," says Sightline’s Brad Plumer:

A few years back I wasted hour after hour playing
with the java settings, and watching "traffic" jams materialize and
melt — just like in real life.  My favorite quirk:  for one
lane-narrowing scenario, I could make traffic flow along beautifully at
40 miles per hour, but seize up like glue at either 20 mph or 60 mph. 
Another fave (and very relevant to congestion pricing debates) was
letting traffic flow along smoothly at, say 1,400 "cars" per hour, and
then increasing traffic volumes to 1,500 — and watching the traffic
jam crystallize within moments.