The emission factors of PM10 and PM2.5 (particulate matter less than 10 and 2.5 m aerodynamic diameter, respectively) for vehicles operated on paved roads in California were characterized by a three-way approach. The primary PM measurement device was a Thermo Systems Inc. DustTrak, a fast-response instrument based on light scattering. These instruments were calibrated against PM concentrations determined from filter collection followed by mass weighing. The first approach was a long-term upwind-downwind program measuring PM concentrations on both sides of an arterial road. In the second approach, we measured PM concentrations directly in a vehicle's wake using an instrumented vehicle with a trailer. The final approach determined the effect of trackout, an upset condition where soil was directly deposited on the road's surface.