In the end of the 1990s VTI and Statistics Sweden (SCB) jointly developed an estimation model for the annual vehicle mileage in Sweden for the period 1950-1997, expressed as vehicle kilometres on road . This model is documented in VTI Report No 439, 1999. Since then, SIKA has annually commissioned VTI to estimate the annual vehicle mileage for different vehicle types in Sweden. Initially, a number of minor modifications were made, but in 2003 there was a major change since data from SCB and Bilprovningen (the Swedish Motor Vehicle Inspection Company) were available. There were two reasons for the model change. One was to make use of the detailed information provided by SIKA/SCB/Bilprovningen and the other to categorise lorries according to total weight instead of maximum load. Before 1990 lorries were categorised based on axle spacing and maximum load. In the new version of the model total weight is used to form the categories. The total weight does not include an eventual trailer.