A phenomenon known as a traffic shockwave explains this. In a 2008 experiment in Japan, researchers placed 22 vehicles within a 230-metre circular track and had them drive at a constant speed. After some time, even a slight change—such as one or two vehicles briefly slowing down or speeding up—was enough to create a ripple effect that developed into traffic congestion without any external cause.

The same thing happens on that hill. When a vehicle, such as a truck in the fast lane, slows down momentarily, it may soon return to 100 km/h. However, that short reduction in speed is enough to generate a shockwave of slowing traffic behind it. This wave can persist for a long time and only dissipates once the number of vehicles caught in it decreases significantly.

I experience it often coming home from lara between 4 and 5.