Well it is not just the collisions, But for people with a lot less powerfull pc (like mine) it is almost imposible to have the game running steady with that amount of players on one point.
What in my opinion is causing the collisions. Don't get me wrong but EP can be busy indeed but so can Calais be and even the Scandinavian ports. But a lot of players are not even going to Rotterdam because of this problem.
If you actually take a look on ets2map.com you see a very dense populated Rotterdam and on other parts of the map it is quite calm.
So how about that traffic gets moving to actually populate a lot more of the empty map space what would balance the unnecessary amount of traffic in Rotterdam and atract players who are actually there to pick up or drop off a load etc.