Such situations are relatively common and because there is no obvious reason or cause, they are called 'phantom traffic jams': it is a traffic jam, but not a real traffic jam... a phantom (a ghost). Often there is only one small event which causes a chain reaction. A vehicle slows down a bit and the next vehicle has to brake a little harder and the chain reaction has to bring the 10th car in line to a complete stop. This chain reaction continues and a real harmonica movement glides on the rest of the highway.
Although you could argue that the driver of the first car should have been more careful, all the next driver also have a share in the event. People are simply not able to estimate distances and speeds correctly. Because of this and because of our relatively long reaction time, the successive cars will have to brake a little harder each time and finally the last one will stop.
Stress and annoyance
Phantom jams do not make for extra large traffic jams, because quite quickly traffic resumes at its normal speed. It is a matter of a few tens of seconds, unless of course, there is an accident. What phantom jams bring is annoyance and frustration: it's an abrupt interruption of the normal traffic flow and the heavy braking and pulling up – especially if we don't see a clear cause, we get nervous.
More invisible side effects are also the result of this way of driving: cars consume more and the less good combustion also means more air pollution. Also, this may have greater wear and tear on brakes and tires.
The hare and the turtle
Although the speed in itself is not the cause of phantom jams, its appearance, but also its occurrence, has more to do with the global attitude of the driver, who can behave as a hare or a turtle. The hare will not only move quickly, but is also convinced that he must try to get through everything and fill the smallest 'hole' there is. As a result, it will swing between all other traffic and have to decrease speed frequently and then accelerate again.
The turtle maintains a more steady rhythm... probably he slows down a little bit, but he will take the time to react to what is happening in front of or next to him. He doesn't necessarily want to be faster, not even slower, but follows the flow of traffic at a safe distance.
The fact that this latter attitude is not only safer but also ensures a greater capacity of the motorway has been proven by 'block driving' in the 70s and 80s. Keeping sufficient distance and all driving at the same speed prevents such phantom jams and ensures greater safety and flow.
Animal wisdom and human ingenuity
Flocks of birds (and also schools of fish) show us the good example every day. They move at high speeds and keep a fixed distance from each other so that they get ahead safely with the whole group without any problems. They look like well-oiled machines that interact with each other in perfect coordination. Unfortunately, we as human beings do not have the same 'discipline', but neither are we able to make the right assessment at all times. We are also more guided by our personal needs and have less that group feeling.
However, technology can help us in the long run. Cars can communicate with each other and make such decisions regarding keeping distance and adjusting the speed to take over the traffic from humans. Adaptive cruise control (ACC) can automatically adjust the speed and distance of the car to the vehicle in front and one is also working on an extension of this system with rear-facing sensors so that thi distance can also be incalculated. Such technology does not interfere with errors of judgement, nor does it have a long response time. This will significantly reduce the distances between cars in the future. Experiments in the Netherlands with self-driving cars and in Belgium with platoon trucks have shown that the tracking distance can be halved by using this technology, without having to reduce the normal speed. The flow improves considerably without the need to build additional roads and lanes.
But in the meantime...
In anticipation that a majority of our vehicles have that technology, we will have to discipline ourselves and play a little more the turtle: not that we have to drive much slower, but that we do have to fit in the flow of the vehicles in front and next to us and keep enough distance. This ensures fewer (phantom) traffic jams and a safe homecoming.