Measuring traffic congestion: a better way

While investigating traffic congestion around the Johan Cruijff Arena (JCA) in Amsterdam, I naively thought that a low recorded traffic speed or flow were signs of congestion in a particular road segment. As I learned, the reality of it is a bit more complicated. We cannot solely rely on flow or speed to estimate if there is congestion or not. A low flow can mean congestion but also that the road is empty. A low speed can mean congestion but also that some people are simply driving slowly for no apparent reasons.

So, how can we estimate congestion?