Ultrafine particles (UFP) have both health and climate impacts and are emitted in large quantities by vehicles. To further our understanding of how high-traffic roadways contribute to urban UFP concentrations, a mobile sampling strategy is developed to measure UFP on a perpendicular transect near interstate 81 (I-81) in Syracuse, New York. Measurements are taken during morning rush hour and non rush hour using a bicycle-mounted TSI Nanoscan scanning mobility particle sizer capable of measuring particles with diameters of 10 – 420 nm. UFP concentrations near I-81 are considerably lower and more variable than reported in previous studies. The spatial decay of UFP with distance from I-81 is examined, and a linear mixed-effects model is generated to determine the variables that influence near I-81 UFP concentrations. Temperature, sampling time, and distance are important predictors of near-I-81 UFP concentrations, but local UFP sources are evident and identification of such sources is explored