Yesterday on the motorway it was raining, sometimes quite heavily. Obviously that means a speed limit of 110kph.
We passed a police car at the side of the road and it looked like the front seat passenger was using a radar device. We were doing an indicated 130 (and were among the slower vehicles 
).
Is the reduction ever enforced, do you know?
No idea, but we might find out in the next few weeks 
My own upper limit all the time. 
I always think about ‘what is rain?’ for the limits? A few drops? At the point you need your wipers? What happens with those silly light showers where they stop and start all the time?
reducing speed during times of rain/heavy rain is to enable people to stop before hitting the vehicle in front/whatever… if stopping is essential.
if the rain has stopped, but the road surface is still slippery… one should still slow down accordingly…
(that’s the brief translation of the official blurb)
I don’t think the police would have a speed trap on an autoroute hard shoulder. It would be incredibly irresponsible, especially in the rain.
I remember when the restriction came in, it was 1981 or 82. If I remember correctly it was after a coach full of school children was involved in very nasty autoroute accident in heavy rain. I don’t know if the rule has been enforced n practice. For cars equipped with adaptive cruse control it’s somewhat redundant.
There are very few fixed speed cameras on the autoroutes so that’s exactly how they do carry out speed checks and may radio ahead for the speeding ticket to be issued after passing through the toll plaza. Otherwise a postal version may be issued.
Plenty of videos of such operations on the internet. one from TF1 HERE
We might have to agree to disagree 
they are not on the hard shoulder. They are in a refuge. The Alpine A110 is nice though.
Don’t forget that (officially) the 110kph limit falls to 100 kph as well.
I’d assume the standard speed cameras are not smart enough to automatically adjust for the weather, but you’d think that if the police are out and about with a radar gun that they’d be enforcing whatever speed limit was appropriate for the prevailing conditions.
Enforcement might depend in part on whether they have met their quotas. Was fined many, many years ago in an almost exactly identical situation, on the A31, and going at same speed, but at the time there wasn’t the “délit de grande vitesse”, maybe these days they only look for that.
We were zapped for being just over the lower limit by a speed camera on the A28. It was barely spitting. A big fat raindrop must have plopped onto its sensor. We pass it regularly and I always curse it.
Is there a legal definition of how much rain constitutes rain for the purposes of adjusting speed limits?
It probably comes under the same jurisdiction of how long to stop at a stop sign 
I did not know it was raining, I thought the driver in front was washing his windscreen
A sensor on my windscreen starts the windscreen wipers automatically as soon as rain is detected. I then turn my lights on, in accordance with the French highway code, and maybe a warning bleep to limit speed could be emitted automatically - on all new.cars, or does such a system already exist?
Yes it is and you were breaking the rules of the road. You deserve to be fined and points.
You are full of the joys of Spring!
The S3 turns the headlights on if the automatic wipers need to make more than the occasional sweep.
I read that as full of the joys of speeding. 
