Effective duration and time‐integrated noise level (exposure level) of the sound of a passing vehicle as measured at a point on a sideline, are modified by directionality of the sound source. The factor of proportionality between the effective duration and ratio of slant range to speed is shown to include the integral of the directivity pattern of the source. Since the effective duration increases in proportion to the minimum slant distance and inversely as vehicle speed, the (time‐integrated) noise exposure level is reduced only 3 dB by each doubling of distance and doubling of source speed. Average (rms‐equivalent) sound level, during a period such as an hour, is equal to the total integrated sound level over the hour minus the duration allowance, 10 log (3600 sec/1 sec) = 35.6 dB. Hence, in contrast to maximum noise level that decreases 6 dB with doubling of distance (plus absorption), average noise level from a single moving source decreases only 3 dB for each doubling of distance of closest approach. A routine is described for obtaining the time‐integrated noise level of a single flyover or the average (rms‐equivalent) sound level from a record of sound level versus time for the times that selected levels are exceeded.