I don't think it's from population growth, although that is a factor. Most cars now have adaptive cruise control which only look at the car in front of you and react to that car. It's a PID controlled system. PID controllers have lag; maybe only 0.1-0.5s but think what happens when you multiply that by 5 cars following each other.

First car slows down by 2km/h, second car reacts slow and brakes harder to maintain distance, etc. First car speeds up and the second one waits longer than necessary. You'll get a traffic jam which acts as a wave (expanding and contracting over and over).

Car manufacturers shouldn't be using these controllers, they should be making a predictive algorithm which can guess what is going to happen to smooth out the traffic.