I know that this may not be in the spirit of adventure that we are all supposed to demonstrate when gracing Johnny Foreigner with our presence but for the past couple of years the French authorities have both announced and demonstrated their intention to clamp down on speeding. This is countrywide and all year round. Is it entirely surprising then that the major roads from the Channel ports to La Sarthe are teeming with radar guns, particularly in light of previous years experiences where a significant minority of English visitors both announce and demonstrate their intention to drive as quickly as possible.
The answer to avoiding eye watering fines is to either keep to the limits or keep off the major routes, unless of course paying a fine is seen as either demonstrating that you have huge cojones or that the frogs are out to stuff the English. Me, I prefer to spend the money on something more enjoyable (beer, vin, more beer, more vin etc. etc.)
JT