The French have tightened up their laws and their supervision of motorists considerably in the last few years. The latest is the new requirement to carry a breathalyser kit in all cars from July 1st.
You need to produce an unused self test kit (no excuses about checking your alcohol level before the gendarmes swoop), marked with the French 'NF' certification standard.
So you should carry two: one to use and one to produce. The fine for not having a kit is 11 euros, though French police are expected to operate a grace period up until November.
One hold-up in introducing the law was that there were just not enough kits manufactured. They've overcome that one, and now you can buy them at ports and on board. P&O Ferries, which carries over a million cars and a million lorries between France and the UK annually, has stocked up with bulk supplies of twin packs.
You should note that the drink driving limit is 50 mg of alcohol in 100ml of blood, which is much lower than the UK's 80mg.
A few tips
You have been warned.
If you're renting a car in France, one of the best options is the Renault Eurodrive Buy Back Car Leasing Scheme