Microplastic pollution of stormwater can be a serious threat to the environment. Gross pollutant trap (GPT) - bioretention treatment trains have been shown previously to treat (inter alia) particulate stormwater pollutants including microplastic particles larger than 100 μm. This study was carried out to investigate whether such stormwater treatment trains also remove smaller 20 to 100 μm sized microplastic particles from highway runoff. Further, it investigates occurrence and concentration of 20 to 100 μm sized microplastic particles in highway runoff and which polymer types they can be assigned to. Volume proportional samples from nine rain events were taken from the incoming highway stormwater, from the gross pollutant trap effluent and the outflow from a bioretention system as well as a non-vegetated sand filter. The microplastic analyses were carried out using μFTIR and FTIR-ATR, which made it possible to detect particles where carbon black was present. It was found that 20 to 100 μm sized microplastic particles are abundant in highway runoff and that their concentrations are highly variable, with a median of 230 particles/L, a minimum of 42 particles/L and a maximum of 8577 particles/L. The dominant polymer types in highway stormwater were Polypropylene (PP), Ethylene Propylene Diene (EPDM) rubber and Ethylene-vinyl acetate (EVA). The treatment train with the bioretention system treated 20 to 200 μm sized microplastic particles significantly better than the treatment train with a non-vegetated sand filter, with median effluent concentrations of 26.5 particles/L and 121 particles/L, respectively. The GPT had no significant impact on the treatment of 20 to 100 μm sized microplastic particles.