Particle Size Distribution (PSD) in highway runoff was monitored in the 2004–2005 rainy season at three highway sites in west Los Angeles, California. PSD was measured for 200 grab samples for 18 storm events. Particles and especially larger particles showed a strong first flush. On average, the initial 20% runoff volume transported approximately 28% total number of particles between 0.5 and 2 μm in diameter, more than 30% of particles between 2 and 30 μm and more than 40% of particles larger than 30 μm. A naturally occurring aggregation was observed with smaller particles and mixing experiments were performed to determine the possible benefits for sedimentation and filtration. Samples composited from grab samples manually collected over the first hour of runoff were gently mixed (G = 38) and small particle concentration decreased by more than 50%. After 24 hours the number of particles with diameter between 0.5 and 7 μm decreased by 51% with gentle mixing and the same size particles decreased by only 14% without mixing. Number of particles with diameter larger than 20 μm increased by 6 and 4.5 times with and without mixing, respectively. Slow mixing can improve sedimentation efficiency by more than 40% for particles less than 20 μm in diameter.