Figure depicting geometry of minimum separation-distance between vehicles for noise emission level measurements is shown. The measurement microphone is at distance D from the center lane of travel, and a line from the microphone to the roadway is drawn perpendicular to the center lane of travel. This intersection is the closest point-of-approach and is also the location of Vehicle 1 (labeled X1), which is traveling to the right. Vehicle 2 (located at X2) is approaching Vehicle 1, and delta X is the minimum separation between the two vehicles.

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