New Speed Limits in France
From 1st July 2018 the national speed-limit on ordinary roads in France (i.e. excluding divided highways and motorways) will go down from 90 km/h (56mph) to 80 km/h (50mph).
This covers around 400,000km of roads that do not have a central reservation or barrier and includes most of the routes departementale (D roads) and a lot of routes nationale (N roads).
Detection will be by unmarked cars operated by a private company, you don't see anything and don't get pulled over for exceeding the limit, you get an automatic fine sent by post to the vehicle's registered keeper/owner.
Crit'Air Stickers (Air Quality Certificate) for France
Crit'Air stickers indicate the emissions standard of the vehicle, it is a round sticker that must be displayed in a prominent position on your vehicle. It is a secure document.
There are 6 different types of certificates. Each certificate corresponds to a vehicle class defined according to air pollutant emissions. Older vehicles covered by less stringent emissions standards are not eligible for a certificate.
For eligible vehicles, the Air Quality Certificate is mandatory for driving and parking in restricted traffic zones (ZCR - French low emission zones) designated by the authorities, such as inside the Paris ring road, in Grenoble, and Strasbourg.
Additionally there are ZPA zones where restrictions on driving within the zones can be introduced on a temporary basis, depending on the level of air pollution measured. In this case the local authorities can implement a ban on driving within the zones for those vehicles that do not meet the required emissions standard.
Information about the certificate and how to order one is available on the official French Government website, in English, here: https://www.certificat-air.gouv.fr/en/
The whole subject of low emissions zones and regulations in Europe is a complete mess, different countries, and in some cases different areas within a country, have implemented completely different standards and schemes to control air pollution. There seems to have been no attempt at standardisation. Residents of individual countries will probably get to grips with the rules that apply in their location, but for tourists and travellers it can be very difficult to understand the different standards that apply in the countries they plan on visiting.
The website https://www.green-zones.eu/en.html has information about all the low emissions zones in Europe and maps to show their locations.