**3.1 Direct measurement method**

The direct method described in the ISO 10847 standard is the approach to be used when the barrier has not been installed yet or can be removed. The method requires measurements before the barrier has been constructed to determine "before" levels and measurements at the same site after construction to determine "after" levels. According to US Federal Highway Administration (FHWA) [17, 39] this method ensures identical site geometric characteristics but also requires equivalent "before" and "after" meteorological and traffic conditions that may be difficult to reproduce. These meteorological
