It is difficult to understand how a five lane highway becomes so packed? It feels as if every car in the city has joined the highway and gave it the snail pace. But what if all the cars keep on moving at a constant speed without ever having to stop (assuming there are no blockades or road accidents etc.)? Theoretically, the traffic jams should never occur! So the main cause of traffic is brakes; decreasing the speed is what initiates the traffic jams.
As the first car stops, the following cars must also stop. Now, even when the first car begins to move, the additional approaching cars, behind the second car, have to slow down (or stop) farther down the road. This congestion keeps on travelling backwards in a wave unless there is enough space for it to dissipate.
Traffic occurs most likely due to more cars entering the highway than leaving it; again assuming construction, accidents, and stalled vehicles aren’t the reason. Brakes have to be applied as more cars enter the crowded highway to avoid collision, thus creating a traffic-wave.
Another reason for traffic jams is abrupt changing of lanes. See the below gif which shows how a lane change by red car creates a congestion wave causing the jam.