The court decision (joined Cases T-339/16, T-352/16, T391/16) of 13 December 2018 and the resulting current adjustment are about conformity factors. Conformity factors are used to determine the permissible deviation of the RDE values when the vehicle is moved by a real driver on a real road from the emission limit value tested under laboratory conditions and are intended to be gradually reduced.
In October 2015, the Commission presented a proposal to implement the new RDE tests. The decision was approved by the European Parliament and the Council in February 2016 In a first step, a temporary compliance factor of 2.1 has been set for all new vehicles for the transitional period until 2020 to take into account both the statistical uncertainty of the test procedure and that of the measuring instruments.
After 2020, a much lower compliance factor (1,5) should apply, which only reflects the uncertainty of the measuring instruments. This margin of error will then be reviewed annually to take into account progress in the accuracy of the portable emission measurement system.
The Commission undertook to review the compliance factor annually with the aim of achieving a continuous reduction. In the light of the latest improvements in metrology, the 4th legal act provides for an initial reduction of the compliance factor from 1.50 to 1.43.