In recent years roadside concentrations of the European regulated pollutant N02 have been leveling, or in some cases increasing, throughout Europe and in the UK. The roadside concentration behavior was unexpected, and was not predicted by air quality modeling due to a poor understanding of typical or 'real-world' f-N02. Various estimations have been made but there are few measurements of real-world f-N02 due to the inherent difficulty of measurement afforded by the ambient reactivity of NOx. In answer to the need for determining real-world f-N02 both a detailed and comprehensive measurement campaign was undertaken in a road tunnel. The Westgate Tunnel, the tunnel in which the research was conducted provided an environment where measurement of primary emissions of NOx could be made, due to the restricted dispersion environment and the removal of the impact of secondary chemistry. Further to this aim, the research employed the modified tunnel rationale where simultaneous measurements from two positions in the bore are made. The net difference between the measurements was considered as primary NOx, and used to quantify f-N02. To improve upon previous research a detailed characterisation of the tunnel was made, to ensure a full understanding of the environment. Consequently, a 23 week measurement campaign was undertaken between January and September 2010. The results exhibited a high degree of statistical reproducibility and all weekdays were combined into a single overall dataset from which real-world f-N02 was quantified. This provided f-N02 ranging between O.10~~:~~ and 0.21 ~~:~~, with a mean value of O.17~~:~~. Our observed f-N02 compared favourably with estimated f-N02 values derived from chassis-dynamometer studies, that are currently used and recommended for use in air quality modelling.