In this paper, we propose a mixed methods quantitative and qualitative approach to capture fully the measurable and less tangible social impacts of transport projects on local people and communities. The approach was used to assess the potential social impacts of a strategic road by-pass project case study in a deprived region of Wales in the UK. The project specifically aimed to stimulate local economic growth and regeneration in the local areas it serves. In a ‘before and after’ case study, we combined fine-grained, GIS-based spatial analysis of secondary datasets with qualitative participative exercises with the local residents of the five communities living adjacent to the road, and interviews with professional local stakeholders. This mixed methods approach significantly enhanced understanding of both the social benefits and disbenefits of the road project. It helped to reveal local concerns that would not otherwise have been apparent from secondary dataset analysis alone. The qualitative studies were also successful in bringing to the table new ‘hard to reach’ voices that had not been heard through the formal consultation and public engagement process. The study revealed that the social benefits accruing to local people from the project could have been significantly enhanced, whilst a number of its locally occurring negative social impacts could have been avoided had social assessment been employed earlier in the decision processes concerning its routing and design. Recommendations to improve the practice and uptake of social assessments at the option appraisal, project design mitigation and post evaluation stages of transport projects are included in the paper.