This may be forehead creasingly obvious to the veterans here, but for any young newbies travelling to France for the first time:
The French drivers, god bless em, will warn you of an approaching speed trap. So if you see an oncoming car flash its lights at you, he's not trying to tell you that the some of the bodily fluids from the cadaver in the boot are leaking on to the tarmac, he's telling you that a speed trap is just up ahead. Nice, really.
H