In Response to Pollution Spike, Paris Temporarily Halves Traffic By Decree
As reported by News.com.au, the city of Paris has implemented a harsh (but temporary) measure for drivers, in response to a surge in pollution: banning cars with even-numbered registration plates from the streets. According to the article,
City mayor Anne Hidalgo had asked authorities to prevent one in every two cars from taking to the capital’s streets and make all public transport temporarily free in a bid to drive down pollution. Only vehicles with numberplates ending in an odd number will be allowed to drive, though exceptions exist for vehicles like taxis, electric cars and ambulances. ... Public transportation is to be free until at least Monday in Paris and its surrounding towns in an effort to force pollution down by coaxing drivers to give up their cars for a few days. Similar emergency measures were last implemented almost exactly a year ago — on March 17 — during a particularly bad spike in the pollution levels.
Yes, but you're missing the crucial point that "No Diesel" is very hard to enforce -- typically diesel commuter cars have only a small badge (if that) to distinguish them from the petrol versions, and the badge is different in appearance and placement between manufacturers and models. By contrast, banning cars based on license plate is very easy to enforce, as they are standard across vehicles and police are already accustomed to inspecting them by habit.
In short, a non-optimal rule that can be enforced is much better than an optimal rule that can't.