That is a mathematical model explaining one type of traffic jam - the "phantom" traffic jam. It can explain a traffic jam on a straight section of highway with no entrances or exits -- but imagine what would happen to that model if a second line of traffic with the same density of cars tried to merge in!

According to a 2005 report for the Federal Highway Administration, the leading cause of congestion is bottlenecked infrastructure.

https://en.wikipedia.org/wiki/Traffic_congestion#Causes

I used to drive 405 North into Bellevue, WA. In the space of 3 miles, 3 highways merged into 405 North. There was always a traffic jam, every morning. It started at roughly the same spot every morning. It cleared up at roughly the same spot every morning. That was slow traffic caused by the infrastructure.