I'd say 40 cm is not enough. It may suffice for those who open the door a tiny bit and then look whether a cyclist is approaching, but those aren't the problema

I'd say 40 cm is not enough. It may suffice for those who open the door a tiny bit and then look whether a cyclist is approaching, but those aren't the problematic kind; rather those who just open the door fully without checking anything, ruining either a cyclist's face, or a car's mirror.