(13) Member States should comply with the emission reduction commitments set out in this Directive from 2020 to 2029 and from 2030 onwards. In order to ensure demonstrable progress towards the 2030 commitments, Member States should identify indicative emission levels in 2025 which would be technically feasible and would not entail disproportionate costs, and should endeavour to comply with such levels. Where the 2025 emissions cannot be limited in accordance with the determined reduction trajectory, Member States should explain the reasons for that deviation as well as the measures that would bring the Member States back on their trajectory in their subsequent reports to be prepared pursuant to this Directive. (14) The national emission reduction commitments set out in this Directive for 2030 onwards are based on the estimated reduction potential of each Member State contained in the TSAP Report no 16 of January 2015 (‘TSAP 16’), on technical examination of the differences between national estimates and those in TSAP 16, and on the political objective to maintain the overall health impact reduction by 2030 (compared with 2005) as close as possible to that of the Commission proposal for this Directive. To enhance transparency, the Commission should publish the underlying assumptions used in TSAP 16. (15) Compliance with national emission reduction commitments should be assessed by reference to the specific methodological status at the time the commitment was set.