We have conducted a study to characterize the airborne and settleable particulate matter generated by vehicles on the road, to evaluate the importance of non-exhaust airborne material from vehicles, and to identify the vehicle types producing various airborne particulate species. Two tunnels -- the Allegheny Tunnel of the Pennsulvania Turnpike and the Detroit & Canada Tunnel -- were utilized to isolate the motor vehicle from other sources of particulate matter. Experiments also were performed at sites in the open. The most important conclusions are: (1) Settleable material predominates in the particulate matter associated with traffic; at Allegheny it outweighed the airborne material by nearly two orders of magnitude. It originates mostly from sources other than engine exhausts. (2) Diesel-engine exhaust is the dominant source of airborne particulate matter associated with traffic, with emission rates ∼1 g/mi, at Allegheny where the average speed is ∼50 mi/hr and Diesel trucks comprised ∼10% of the traffic. (3) Carbon is the most abundant element in the airborne particulate matter from traffic; it comes primarily from trucks, and much of it is present in the form of saturated aliphatic hydrocarbons. (4) Airborne particulate matter from traffic has a large surface area -- up to 30 m2/g, or as high as 90 m2/g after removal of extractable organic material. (5) Airborne particulate matter generated by automobiles at 50 mi/hr is <0.1 g/mi, mostly from the exhaust. At least 20% of this (in 1971) was Pb, of which automobiles are the primary source. The Pb g/mi emission rate exhibits a decline during the 1971-1974 period, in keeping with the trend in gasoline Pb levels. Total Pb emissions at 50 mi/hr represent only about half of the Pb consumed. More than half of the Pb emitted is settleable, with a size range centered around several hundred μm. Conclusions regarding Br/Pb ratios, Zn, Ba, sulfate, and particle size of the airborne and settled material and certain chemical components are presented.