MDOT defines a traffic noise impact:
As approaching 1 dB(A) (A-weighted decibel) of the FHWA Noise Abatement Criteria (NAC), [for example: for NAC Category B (residential) a design year sound level that is equal to or greater than 66 dB(A) would be considered an impact], or;
When design year predicted noise level is 10 dB(A) higher than existing.
FHWA Noise Abatement Criteria (NAC)