The amount of vehicles a road can handle before becoming congested is dependent on the safe distance needed between cars at a particular speed, the reaction tim

The amount of vehicles a road can handle before becoming congested is dependent on the safe distance needed between cars at a particular speed, the reaction time of drivers, and the consistency and coordination of driving styles. You forgot the most important factor: the exits. If the exits are congested because capacity is lower near the destination, there's nothing you can do in the road. And most jams are caused exactly by this.