^Trivia: Congested traffic, traffic jams, etc ALWAYS occurs when
the density of cars reaches 40 vehicles per mile. Why?
No matter how smoothly traffic is flowing, once it reaches 40
cars per mile and beyond, the little speed up/slow down that
everybody does when driving quickly add up at the front of the
line, causing a temporary mini jam. That leads to a minijam
behind it as the 1st bunching of cars clears up, and so on like
a catepiller moving down the line.^
