THE STRENGTHS AND WEAKNESSES OF TRAVEL DEMAND MODEL SYSTEMS FOR ASSESSING EMISSION REDUCTION POLICIES

Systems of models which predict various aspects of travel behaviour - frequency of travel, choice of destination, choice of mode, choice of route, etc. - are now often applied to predict the influence of transp ort policies. The forecasting horizon for such studies is often as lon g as 15-25 years into the future. In recent years, the environmental a nd energy impacts of private travel have become vital issues in transp ort policy assessment. The main question addressed in the paper is thu s what travel demand model systems can and cannot yet tell us about lo ng-term changes in emissions from private transportation. The paper fi rst outlines the typical structure, inputs and outputs of travel deman d model systems. Then, two European case studies are described - one i n the Netherlands and the other in Norway. In both cases, predictions of future emissions levels were key results. These studies illustrate both the strength of such models in providing a detailed and realistic treatment of the demand for travel, but also some current weaknesses with respect to predicting fuel use and emission rates. The final sect ion discusses two particular areas of modelling which can address thes e weaknesses. The first area is that of vehicle purchase behaviour. Fu rther progress in this area will allow better forecasts of the makeup of the car park in terms of the fuel use and emissions characteristics of the vehicles. A second area is that of driving behaviour, as might be influenced by, for instance, lower speed limits, in-vehicle inform ation systems or greater environmental awareness.