New vehicles must pass a series of type approval tests. Car emissions are tested in a laboratory on a rolling road as the car is driven over a standard test cycle. Petrol cars achieve the emission limits in the laboratory by a wide margin; diesel cars less so. However, remote sensing and portable emission measurement systems have shown that emissions from diesel cars are much higher when driven on the road than under laboratory conditions. This is not the case for petrol cars, which achieve similar results in the laboratory and on the road and outperform diesel in both environments. There has been little improvement in NOx emissions from diesel vehicles (both heavy and light duty) until very recently. For example, the latest generation of diesel vehicles have been shown to have lower on-road NOx emissions than earlier generations; with Euro 6 cars, for example, having about half the emissions of Euro 5 cars.