In the context of stormwater quality modelling, an in-depth understanding of underlying physical processes and the availability of reliable and accurate mathematical equations, which can replicate pollutant processes are essential. Stormwater pollutants undergo three primary processes, namely, build-up, wash-off and transport, before accumulating into receiving waters. These processes are expressed mathematically by equations in stormwater quality models. Among the three processes, wash-off is the least investigated. This paper presents the outcomes of an in-depth investigation of pollutant wash-off processes on typical urban road surfaces. The study results showed that a storm event has the capacity to wash-off only a fraction of pollutants available and this fraction varies primarily with rainfall intensity, kinetic energy of rainfall and characteristics of the pollutants. These outcomes suggest that the exponential equation commonly used for mathematically defining pollutant wash-off would need to be modified in order to incorporate the wash-off capacity of rainfall. Consequently, the introduction of an additional term referred to as the 'capacity factor'C(F) is recommended. C(F) primarily varies with rainfall intensity. However, for simplicity three rainfall intensity ranges were identified where the variation of C(F) can be defined. For rainfall intensities less than 40 mm/h, C(F) varies linearly from 0 to 0.5. For rainfall intensities from 40 to around 90 mm/h, C(F) is a constant around 0.5. Beyond 90 mm/h, C(F) varies between 0.5 and 1.