It doesn't matter. The car joining the lane must do so in a way that doesn't cause the traffic in the land they are joining to break.Or the cars in that lane are speeding or tail-gating?
I know, but that's the way it is.I find it remarkable that vehicles attempting to join a motorway, from a limited joining 'window' have no 'right' to do so as such and vehicles already on the motorway have no responsibilities at all in terms of accommodating them, other than general guidance about driving with consideration to other vehicles.
If the cars in the lane to the right are driving dangerously then cutting in in front of them isn't a good idea.