Thats a good point regarding reliabilty RobboC.
If the French government enforces this rule, it is possible for someone to drink drive over the limit, after taking their own breatherliser test. Which was unknowingly an inaccurate reading. The outcome of the enforcement is potentialy lulling people into a false sense of security, thinking that they are within the limit. The oposite senario is, if breatherlisers were not carried, the person would not use one and would have to make their own judgement. More times than not (if the person is not too far gone of course) sense will override their doubts and they will not drive.
personally I don't ride or drive with any alcohol level, so I will never have that problem.