This paper argues that existing models of urban concentrations are incomplete unless grounded in the most fundamental aspect of proximity; face-to-face contact. Face-to-face contact has four main features; it is an efficient communication technology; it can help solve incentive problems; it can...

There is a substantial empirical literature quantifying the positive relationship between city size and productivity. The paper draws out the implications of this productivity relationship for evaluations of urban transport improvements. A theoretical model is developed and used to derive a...