*3.1.3 Measurement period*

A range from 2 to 30 min is the usual sampling period for measurements. When weather conditions are fluctuating, longer sampling periods, such as 1 or 24 h, could be more accurate [17]. It has been suggested [40] that 2-min measurements, as specified in the standard, are too short for a stable and reliable evaluation of sound pressure levels. The optimal measurement periods are 15–30 min, as longer periods

#### **Figure 4.** *Alternative positions for reference microphones—blue circle—(based on [17]).*

**Figure 5.**

*Measurement microphones in the study of Anfosso-Lédée et al. [40] (above) and Parnell et al. [41] (below).*

would possibly introduce atmospheric changes [40]. However, other researchers [41] established a 10 min measurement period.

The FHWA suggests avoiding measurements when wind speed exceeds 17 km/h or while raining since raindrops generate noise and tire noise increases on wet pavements. The Agency also recommends avoiding measurements when traffic flow is congested since the traffic noise level will be lowered, making it more difficult to evaluate the IL.
