The Philadelphia Roadway Study was conducted to determine the horizontal and vertical extinction profiles of particles generated by vehicles, including fine (0-2.5 micrometers), coarse (2.5-15 micrometers), and total (0-15 micrometers) size fractions in the area surrounding a major roadway. Particulate concentration profiles were characterized to a horizontal distance of 175 m downwind and to a height of 15 m above ground (at five and 25 m downwind) with the primary objective of estimating the influence of a nearby roadway on ambient particulate mass concentration levels. The dilution gradients for concentrations of fine, coarse, and total mass and fine lead particles at two m height are described empirically as a function of downwind distances from the highway using logarithmic equations. Wind speed had a substantial influence on the particulate mass concentration levels both upwind and downwind while the limited range of traffic densities, i.e., 2119-3906 vehicles/hour during the experiment, did not. Fine particulate mass at a height of 2 m at five m downwind was elevated 18% above background, coarse mass 45%, and fine lead 718%. Exponential equations relating concentration to the reciprocal of height and distance were fit to the data for both ground level and vertical concentration. Concentration isopleths were constructed to map suggested siting constraints.