My wife has a blue handicap placard (USA) --- are these honored in Europe? If yes, how? If not, are there special parking rules for h
andicap persons?
You might want to change the title of your Thread to more closely reflect your question. Having a name listed as the title provides no information for people who may be able to provide answers.
Regarding your question, I don't have any information on whether Europeans will honour handicapped signs issued in North America. Hopefully one of the others will be able to provide some information on that.
In our experience it is not honored. Others may have different experiences though. Each area (country and/or region of a country) probably has their own permits. Our young child was in a wheelchair for 4 months a few years back and we were able to obtain a handicapped permit for a length of time prescribed by the doctor. Keep in mind, that doesn't mean you'll find handicapped spaces as plentiful as in the US, especially in older towns. You may have to drop the handicapped person at the door while the other person parks elsewhere. Contact each of the towns--their City Hall equivalent-- you want to visit and ask them their rules.
As Kathy mentioned, handicap placards are not particularly useful unless you're shopping for groceries. Most museums and monuments do not have immediate on-site parking. At best, you'll get a parking spot near the edge of a lot or near the entrance to the garage, but you'll still have to walk a distance to the site of interest.