The Federal Highway Administration's “Highway Noise Abatement Criterion” uses the peak hour equivalent sound level for setting limits in highway noise impact studies. In studies conducted by Cavanaugh Tocci Associates, Inc., it has been observed that the FHWA criterion is generally adequate during the day when the traffic mix is such that traffic sound levels are relatively uniform with time, i.e., when the difference between the intrusive sound levels (defined here as the 1 percentile level,L01) and the background sound levels (defined here as the 90th percentile level,L90) are less than 10 dBA. At night, although the FHWA traffic noise criterion remains satisfied, the difference between theL01sound levels produced by truck passbys and theL90sound levels significantly exceed 10 dBA. Once this 10‐dBA margin between theL01and theL90sound levels is exceeded, community response to traffic noise is affected. This large difference between the intrusive and background sound levels seems to explain the “but I can still hear the trucks” syndrome. This experience suggests that highway noise acceptability criteria and environmental impact assessments should also include a limit on the difference between intrusive and background sound levels. Preliminary criterion limits based on studies to date are described.