English: http://vision-traffic.ptvgroup.com/en-uk/products/ptv-vissim/ - This is the one of two simulation visualizations to demonstrate the effect of speed harmonization (speed limits) in certain circumstances. The other simulation visualization can be found here: https://www.youtube.com/watch?v=Ga3_ifSC5dk

In case of a situation where a slow vehicle turns onto the fast lane right in front of fast moving cars, the car drivers break increasingly stronger as a result of a chain reaction. This behavior is caused by the time delay before braking due to the reaction time of the drivers, especially if they travel below their security distances. As a result a driver needs to brake stronger than the preceding vehicle. The effect is even higher if drivers are traveling at high speeds and/or high speed differences.

As a result, the vehicle speeds drop dramatically, and they can even lead to a total standstill. A queue builds up even though there is no accident, ramp, construction site or any other lane blockage nearby. To subsequent drivers it appears that there is a queue "out of nowhere". This phenomenon is called "phantom jam", "ghost jam" or "shockwave" (in German it is called "Stau aus dem Nichts" oder "Phantomstau").

A speed limit helps reducing the overall speeds and their variation (i.e. the drivers all drive with more similar speeds) and at lower speeds the security distances are also much lower. As a consequence, braking activities (especially strong braking activities) are much less frequently triggered and hence less ghost jams occur.

These simulations and visualizations were done using the microscopic traffic simulation software PTV Vissim.