BE, that's a big 10:4 big buddy come on.
Dick, in the ideal world you are right but a lot of drivers are ignorant feckers at the best of times. If the box is positioned so that there is a cars length the other side between the edge and the merging traffic, then all well and good but most of them go right up to where cars come out. There is one place I know of north eastish where you have to enter and stop on the box to cut off the ignoramuses coming out. At busy times of the day, you would sit there for an unfeasible amount of time if you wanted to make a lawful manoeuvre. I haven't put the location deliberately because it is not enforced currently and I don't want anyone getting any bright ideas.