Road traffic noise exposure leads to annoyance and impairment of quality of life and may impair health. If this association is causal, a reduction in noise exposure should result in a reduction in noise annoyance and improvement in quality of life. This study examines whether the reduction in road traffic noise following the introduction of a bypass leads to reduction in noise annoyance and common mental disorder and an improvement in quality of life. Repeated measures field study with intervention in three small towns in North Wales, UK. Participants were residents 16 to 90 years living in areas of high or low exposure to road traffic noise. At baseline there was no difference in annoyance, quality of life or common mental disorder between traffic noise exposed and quiet areas. There was a small reduction in noise exposure (2-4 dBA) with the opening of the bypass. There was no reduction in noise annoyance and no change in levels of common mental disorder and quality of life following the introduction of the bypass. Traffic noise reduction associated with the introduction of the bypass was not associated with measurable changes in quality of life or common mental disorder. This study suggests that reduction in traffic noise level of 3dB or less is insufficient to influence annoyance or mental health. However, the methodological difficulties of the study limit the conclusions that can be drawn on whether there is a causal effect of noise on common mental disorder.