(1) Noise measurements shall be made at a test site which is adjacent to and includes a portion of a roadway. A microphone target point shall be established on the centerline of the roadway, and a microphone location point shall be established on the ground surface at a distance of fifty feet from the microphone target point and on a line that is perpendicular to the centerline of the roadway and that passes through the microphone target point. The microphone shall be placed such that it is at a height of not less than two feet and not more than six feet above the plane of the roadway surface. The test area shall include an open site within a fifty-foot radius of both the microphone target point and the microphone location point. The test site shall be essentially free of large sound-reflecting objects.

(2) Noise measurement conditions shall be as follows:

(a) Noise measurements may only be made if the measured average wind velocity is twelve miles per hour or less. Gust wind measurements of up to twenty miles per hour shall be allowed;

(b) Measurements shall be prohibited under any condition of precipitation, but measurements may be made with snow on the ground. The ground surface within the measurement area shall be free of standing water; and

(c) Road conditions shall be such that they would not cause a motor vehicle to emit irregular tire, body, or chassis-impact noise.

(3) In accordance with this section, a measurement shall be made of the sound level generated by a motor vehicle operating through the measurement area on the traveled portion of the highway within the test site, regardless of the highway grade, load, acceleration, or deceleration. The sound level generated by the motor vehicle shall be the highest reading observed on the sound level measurement system as the vehicle passes through the measurement area.