Gaseous and particulate pollutants are produced in large quantities from road vehicles. These traffic-related air pollutants can infiltrate the car-cabins exposing drivers and passengers to toxic substances with serious adverse health effects. The infiltration process depends on many parameters, those from outside of the vehicle (e.g. traffic intensity, meteorology, road type) and those depending on the vehicle itself such as the ventilation and/or the vehicle speed/age. In the present study, we undertook on-road measurements to assess the impact of ventilation and vehicles spacing (proximity effect) on in-cabin to on-road (I/O) concentration ratios of nitrogen oxides (NOx) and ultrafine particles (UFP). The novelty of the present work is that two successive vehicles have been used to circumvent the influence of the emissions of the upstream vehicle on the downstream test vehicle. We show that the distance to the upstream vehicle influences significantly the (I/O) ratios. Overall, the results show that car cabin is less polluted when the ventilation is set to recirculation mode and keeping a distance to the upstream car corresponding to twice its height in non-idle conditions.