City mayors from around the world are starting a car scoring system aimed at cutting air pollution on city streets.

The mayors of Paris and London who head the C40 Cities organization are working out a system to score new cars based on their real-world emissions and impact on air quality. Paris mayor and C40 Cities chair Anne Hidalgo and London mayor and vice chair Sadiq Khan believe the program will help consumers make better choices once informed on the environmental impact of their cars.

They also see the car scoring system setting a standard to pressure automakers away from exploiting loopholes in their vehicle labels and emissions reporting methods.

Mayors at other C40 cities will be working to create and support the global scoring system. So far, the list includes Seoul, Madrid, Mexico City, Milan, Moscow, Oslo, and Tokyo.

“For too long, some vehicle manufacturers have been able to hide behind inconsistent regulation and consumer uncertainty about the damage their cars are causing,” Hidalgo said. “This announcement is a wake-up call to car companies that they need to act now.”

The European Union’s testing has only included fuel efficiency and CO2 emissions, and are based on laboratory tests, according to C40. Actual emissions out on the road can be up to 15 times greater. The new program would issue scores for each vehicle after testing during real-world, on-road conditions.

The data will be accessible to the public trough dedicated websites.

Paris and London have committed to having their programs in place by the end of this year.

While leaving out automaker names, C40 points to the Volkswagen diesel emission cheating scandal and other automakers that have fallen under investigation as highlighting the vulnerability of the EU’s current emissions reporting system. Governments are also blamed for using testing methods that automakers can manipulate to conceal the real-world levels of toxic emissions.

“Some diesel cars that meet the EU’s highest environmental standards, known as Euro 6, in reality release more nitrogen oxide and nitrogen dioxide than a modern heavy duty truck. These NOx and fine particle pollutants are particularly important, as they are amongst the most damaging to human health,” C40 said.

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C40 Cities membership now includes more than 90 cities, more than 650 million people and about a quarter of the global economy. Started as C20 with 18 members in 2005, it eventually changed its name to C40 when there were that many active member cities; the group seems to be sticking to C40 even though its city membership has more than doubled.

New York’s former mayor Michael Bloomberg previously served as chair and now heads up the board. He’s been a vocal advocate of cities playing an integral part in a global strategy to address climate change, now serving as a U.N. envoy for cities and climate change.

“These new vehicle scorings will empower consumers to make informed choices that protect public health and the planet,” Bloomberg said. “This is a great example of how the same steps that improve lives also fuel progress against climate change.”