This paper is concerned with the links between agglomeration, productivity and transport investment. If improvements in transport systems give rise to changes in the mass of economic activity accessible to firms, for instance by reducing travel times or the costs of travel, then they can induce positive benefits via agglomeration economies. The paper presents empirical results from an econometric analysis of the relationship between productivity and accessibility to economic activity for different sectors of the UK economy. The results show that agglomeration economies do exist and that they can be substantial, particularly for services. Furthermore, the effect of agglomeration externalities is not trivial when considered in the context of transport appraisal. Initial calculations typically indicate additions to conventional user benefits of 10%-20% arising from increasing returns to economic mass.