It comes to what is considered safe driving ,someone who reacts defensively shouldnt be blamed but they are in most cases .If they make that descion they just have to do it safely when doing so .Its pretty cut and dry when to avoid an accident but many conditions can be looked at ,like speed .If it looked unsafe to them then they will act according to what they witnessed ,cant fault them for that reaction , motorcyclist get involved because of the speed scenrio all the time .The term unsafe lane change is what would eventually be the deciding factor if an accident was to happen in this case.If the officer at an accident scene finds fault with a driver it generally was investigated thouroughly before he makes that desicion.DEFENSIVE DRIVER "I had an accident from driver coming into my lane".OFFICER ,"Did that driver give you amply room in front of you and or in back of you? DEFENSIVE DRIVER ,"No. " Unsafe lane change . That is if he or she had stayed and maintained their lane.Unless they were taped or hit in anyway ,If they moved from their lane then got involved in another accident to avoid the one potential accident most state laws look at it as their fault .Unsafe lane change .If its a Semi truck your avoiding or certain death from a head on I certainly would head to the ditch and take my chances their ,but if its someone coming into to your lane unsafely by law maintain your lane and brake accordingly if needed.If you were to Ask me how I know ,a texting teenager found my lane oh so inviting one lovely day.