Derivation of Link-based Marginal Social Cost Based Charges in Very Large Assignment Models

Derivation of Link-based Marginal Social Cost Based Charges in Very Large Assignment Models


C Lindsay, WSP Policy & Research, UK; G Gaunt, Department for Transport, UK; I Elston, Department for Transport (formerly WSP Policy & Research), UK


The principles of marginal social cost based pricing are well established, but the application in path-based assignment models is more challenging. This paper reports on the implementation of such an approach using the GB National Transport Model.


The economic principles underpinning marginal social cost based pricing are well established. Recent studies have also demonstrated the benefits of such a pricing regime with the aid of traffic models that mimic traffic reassignment effects with link-level elasticity-based demand responses. However the application of the principles to orthodox, path-based assignment modelling is more challenging. The complexity of network effects, and long run-times associated with large assignment models, as well as equilibrating with a travel demand model, makes the external derivation of equilibrium link prices computationally expensive.

This paper reports on recent work, conducted jointly with the UK Department for Transport, to internalise the calculation of link prices into the equilibrium assignment layer of the GB National Transport Model (NTM). The modelling software was modified to derive equilibrium flows, speeds and charges that reflected the marginal social cost of travel within the core assignment algorithm. The link based prices took account of congestion impacts, variation in the valuation of travel time savings, and changes in emissions and accident rates. The model differentiated travellers by income / value of time into separate user classes.

The technique was tested successfully on a 220,000 link model of the GB road network for 10,000 zones, and then iterated to equilibrium within the wider travel choice modelling framework provided by the NTM.

This paper sets out the approach adopted, outlines the modifications to the assignment modelling software and explains how the technical issues were addressed to produce an operational modelling system to investigate the impacts of marginal social cost based pricing at the national scale.


Association for European Transport