The balance between practicality and uncertainty in the SPB method: How many heavy vehicles do we need to measure?

The statistical pass-by (SPB) method is an internationally standardized method (ISO 11819-1:1997), designed to assess the acoustical properties of pavements. Although the method is attractive for several reasons (such as low investment, measuring of the overall vehicle noise, representativity of the result for actual noise exposure of the dwellings along the road etc.), it also has some important inconveniences and one of the main is the sometimes excessive duration of the measurement. A factor which is responsible for the excessive duration is the requirement to monitor a minimum of heavy vehicles (at least 80 in the current standard). The ISO 11819 standard is currently under revision and one of the requirements that the working group considers changing is to require a lower number of heavy vehicles to be monitored per measurement. In this paper we will discuss the consequences of this on the accuracy of the measurement result. Another problem perceived by the users of the method is the sometimesdifficult assignment to an HV category and this issue will be discussed as well.