Running the procedure "HBEFA-based emission calculation" leads to a warning:
"Emission factors were requested that are not available in the Handbook of Emission Factors (HBEFA). They are listed in the message file."


HBEFA defines only a subset out of all possible combinations of attributes, whereas in a Visum model any combination might be possible. To avoid the message only use the combinations which are available for the HBEFA calculation.
See also the manual and the document
c:\Users\Public\Documents\PTV Vision\PTV Visum 15\Examples\HBEFA_Emissions\HBEFA_Emissions_Desc_ENG.pdf
on page 3:
"After running the procedure, look at the trace file for messages. VISUM reports every link for which the traffic situation in the sense of HBEFA 3.1 and the provided free flow speed do not match,..."


1) Check the total share of segments which do not have emission factors is below a certain threshold, like 1%.
2) Check whether such segments are typically Euro-0 class, i.e. are likely to disappear or exist in insignificant numbers.