The introduction of changes in the type approval testing procedures for vehicles is also a matter of reducing the problem of exceeding the limits specified in standards. The previous laboratory studies of vehicles’ emission did not contain information on the actual engine operating conditions. Particular attention was paid to the comparative study of engines with spark and compression ignition in the context of nitrogen oxides emissions. The reason for this consideration was a significant discrepancy in the size of the engines for vehicles with a similar curb weight. The article presents the assessment of the vehicles’ ecological indicators in real traffic conditions according to the latest proposals for Real Driving Emissions testing. It has been shown that despite the compliance of the vehicles with emissions approval limits in terms of nitrogen oxides performed in the study on a chassis dynamometer, they are – especially for vehicles equipped with compression ignition engines – exceeded in actual operation. It proves that special attention should be paid to compression ignition engines, which pose a threat in terms of nitrogen oxides emissions.