PARIS (AP) — The mayors of Paris, London and Seoul are creating a system to grade vehicles based on their emissions in real-life conditions, after growing uncertainty about automakers’ efforts to skirt rules intended to limit pollutants.

Meeting Wednesday in Paris, they said the idea is to provide transparency so consumers know how much a car pollutes and push for change by using the market force of the world’s major cities.

London Mayor Sadiq Khan said the plan will push automakers to build cleaner vehicles faster. In a statement, Paris Mayor Anne Hidalgo said current regulations in the EU and elsewhere left too much wiggle room and had lost credibility.

Volkswagen is accused of cheating emissions standards on nearly 600,000 diesel vehicles in the U.S., and French automakers are under investigation as well.