This study investigates the contribution of road deposited sediment (RDS) to clogging and the operational lifecycle of Highway Filter Drains (HFDs). RDS samples were collected from 9 Scottish trunk roads and fractionated into grain size classes to determine their particle size distributions (PSD). Results show that RDS PSDs, and the percentage of each grain size fraction, are highly variable. However, despite being collected from different trunk roads, PSD trends are similar, with individual RDS particles ranging in size from <63 μm to >10,000 μm. Medium sand, coarse sand, fine gravel and medium gravel make up 84.1% of the total particle mass concentration, with particles >1,000 μm mostly mineral or asphalt. The study also reveals that the dynamic nature of a trunk road catchment dictates that grading envelopes are essentially instantaneous values. These findings indicate that large particles from the road surface, contribute to clogging and have the potential to reduce the operational lifecycle of HFDs. The study also demonstrated that assuming a single RDS PSD profile for filter drain asset management purposes is unlikely to be representative of a trunk road catchment profile.