We also have this law in the uk. You can be tested and if you are found postive you get banned from driving. To my knowledge it hasnt happened to no anyone yet. Its one of those laws which is there but only gets used in the case of a severe accident (checking to see if drivers were intoxicated). It definately wont be as common as the breathalizer, so wouldnt worry about it too much. Also it is digsuting how they try and put people in jail who arent criminals, but you know what governments are like !