I've learned that in motorcycle riding lessons. You have to constantly defend your safety bubble. If you let drivers see a gap that looks big enough they will try to squeeze through it and will push you off the road.

Yep, it's the same here in the UK. Years ago, when I did my bike test, the examiner would test the gaps I was leaving, so if I indicated to turn right, he would attempt to gently push his way into the gap on my right to make sure that I wasn't leaving room for someone to try to squeeze past.

Obviously he did it in a safe way to make sure that I would be safe on the road on my own :)