Ask HN: What's the best way to detect whether a road is jammed?

I have a webcam looking at a particular narrow bottleneck of road near my house. It's usually either free flowing, or completely jammed for ages, most often due to two wide cars not being able to get past each other leading them to have to reverse and do some weird manoeuvres.

I want to build a system which will output whether it's jammed or not. What would be the best path to go down for this?