What do you mean by "force their way in", Mike? Surely there should always be room between vehicles for another to fit in without drama? There's an exception for HGVs of course whose drivers are on a mission to prevent anything getting between their vehicles, especially onto exit slip lanes. How often on heavily trafficked motorways/dual carriageways do you see vehicles so close together that there's little opportunity to slide safely between vehicles to join the lane to your left? Personally I would rather abort an exit than risk "forcing my way in", but most drivers would perform the latter. It's the same with lane closures when nearside traffic deliberately obstructs merging vehicles.
Maybe the ones preventing 'zipping", i.e. those to the left, are equally to blame. As with many such problems it's often a joint effort.
