This paper summarises Surrey County Council's technical appraisal of the proposed widening of the busiest section of the M25 between Junctions 12 and 15. Surrey's M25 traffic forecasts were based on a very large (450 zone, 7000 link) multimodal subregional transportation model. The model assessed highway junction capacities in a conventional manner (albeit over a far wider area than normally attempted), but also examined the impact on M25 traffic flows of local public transport initiatives, motorway traffic management, and motorway tolling. Peak hour forecasts produced by the model showed marked differences from the NRTF (National Road Traffic Forecasts) based forecasts used by the Department of Transport when dual three lane link roads were first proposed. The implementation of area wide matrix capping was of central importance to Surrey's M25 forecasts. The paper therefore describes the methodology and results of the capped forecasts and discusses both how forecasts of flows on motorways and the resulting pattern of journey lengths were affected. The methods and results of modelling motorway speed limits and motorway tolls in a congested network are summarised. Finally the geographical scope of the modelling provided new insights into the issue of whether increased flows on new roads were attributable to induced traffic or were really reassigned flows from further afield.
Abstract