Density of traffic (k) is the measure of the vehicles per length of roadway. The letter k is used because it comes from the German word koncentration. When you are the only car on the road, the density of traffic is low. When there are lots of cars on the road, the density is high. The density of traffic indicates the Level of Service, which is a grade that traffic engineers apply to roads, and ranges from A to F . Just like your college transcript, from the driver’s point-of-view A is better than F.
The maximum density, called “jam density”, occurs when vehicles will line up end to end, and none can move until the car in front moves. If cars were on average 5 meters long, and literally “bumper-to-bumper”, there could be 200 cars per kilometer per lane. In practice cars are longer, and they are only figuratively bumper-to-bumper, so jam density is more like 200 cars per mile per lane or 125 cars per km per lane. But as the saying goes, “Your Mileage May Vary”, and this result depends on many factors. At jam density, traffic does not flow.