Reduced highway speed limit due to magnetic field At some point on the "Autoroute de la Maurienne", a French highway where the speed limit is normally $130~$km/h, the speed limit is reduced initially to $110$ and then to $90~$km/h. The informative panel reports "champ magnétique", that is magnetic field.
Here is a street view of the first signal:
It is not clear if the field comes from the power lines or from some equipment in the neighboring buildings, but I have been wondering what kind of physics justifies that. Is a very strong field expected to interact with the systems of the vehicle? Can reducing the speed by $30~$% really make a difference?
This question is also related: Cyclist's electrical tingling under power lines but in that case, passing through at $90~$km/h would probably be pretty bad!
 A: I think I have found an explanation:

Traverser trop rapidement le champ magnétique autour des cuves d’électrolyse pourrait perturber le fonctionnement des pacemakers et autres défibrillateurs. C’est sans doute aller très loin dans l’application du principe de précaution.

There are some large aluminum manufacturing facilities there, which use electrolysis to make the metal. Around this equipment, there are fairly strong magnetic fields; when you pass through such a field quickly and you are wearing the wrong kind of pacemaker, the argument goes your life support system might go out of control.
My translation of the quote:

"Traversing the magnetic field around the electrolysis tanks too quickly might perturb the functioning of pacemakers and other defibrillators. No doubt that is an extreme application of the principle of precaution."

Clearly, there are regions of strong magnetic fields there: they continue

...constater que tout près des trois cuves expérimentales ultramodernes, alimentées par un courant de 500 000 ampères, il faut forcer pour faire pivoter, dans sa main, un simple barreau d’acier.

"In the vicinity of three ultramodern experimental tanks, fed by a current of 500 kA, you need to use significant force to turn a simple iron bar in your hand".
In other words - eddy currents are generated, and these in turn produce torques that resist motion. Such currents might affect pacemakers (the phenomenon described is reminiscent of effects you feel around MRI magnets). But by the time you get to the highway, it's extremely unlikely these effects are still as big as claimed... magnetic field are at best dipoles, with inverse cube drop-off with distance.
