Check the stephenson-siemens game - there is a dual lane motorway from Turningford to Church Benmere, and aboul half-way a single lane road splits off, crosses the railway and goes to Church Benmere too. Nobody uses that single lane road. If you check the details, or minimap, you'll see congestion > 400 %, but only 5 cars last month.
May I ask Freddy Hayward to look into this? The current system for measuring congestion is one that he wrote, and it will take far less time for him to debug it than it would for me to debug it, since I should need to spend a very long time learning exactly how the current system works to do that.
I would suspect it to be an anomaly such as a handful of cars having to wait a long time to merge into the motorway. These irregularities usually disappear after a month or so.