This paper presents near roadway light-duty vehicle (LDV) dispersion and ultrafine-particle (UFP) emission factors (EFs) as a function of vehicle flow rate, speed and mode of operation (free flow and congestion) using 300 5-min measurements of carbon dioxide (CO2), UFPs, meteorology and traffic conditions near a LDV only roadway. Measurements were made between 2014 and 2018 on 20 days for 2 to 3 hours per day. Near roadway LDV dispersion was estimated using a vehicle emission model (MOVES) for CO2 EFs, measured CO2 concentrations and vehicle flow rate. EFs for UFPs were estimated based on the calculated CO2 dispersion. This eliminated the need to rely on models to estimate dispersion. Results indicate that the near roadway LDV dispersion is a two-stage process. When there was a small number of vehicles (< 10,000 veh h–1), the vehicle induced dispersion was not continuous (unsteady-state conditions). Under unsteady-state conditions, the rate of LDV dispersion near the roadway increased from 2 to 6 m2 s–1 as the number of vehicles increased. For larger vehicle flow rates (> 10,000 veh h–1), the dispersion was continuous (steady-state continuous mixing). Under steady-state condition, the rate of dispersion was near 6 m2 s–1 and did not increased with additional vehicles. Dispersion models failed to response to unsteady-state conditions and predicted constant dispersion. The UFP EFs varied from 0.5 × 1013 to 1.5 × 1013 pt km–1 veh–1 for LDV (3x). The increase in UFP EFs was related to increase in vehicle flow rate and speed. The outcome of this study provides important EFs for urban planner to quantify air quality near roadway due to UFPs.