VTI has observed seat belt use in a number of towns in Central Sweden since 1983. In the original selection of measuring sites, the emphasis was placed on heavily trafficked roundabouts with both through traffic and local traffic. The intention from the beginning was to study passenger cars with back seat passengers since it was expected that these would be affected by a seat belt law a few years later. The observations last year were made over a total of nine days in August-September 2005, approximately one day per measuring point. The total number of vehicles observed was over 50,000. Compared with previous years, the programme has been reduced by about 20%. When VTI commenced its measurement series in 1983, seat belt use by adult passengers in the back seat was a modest 10%. It increased in one jump to 60% when the law on seat belt use by adults in the car as a whole came into force on 1 July 1986. Today, more than 70% of all adult back seat passengers use a belt. The corresponding level for children in the back seat has been consistently about 10-15 percentage points higher. In 2005, 93% of all children in the back seat wore a seat belt. Over the whole observation period, 1983-2005, seat belt use by drivers and front seat passengers has been at a considerably higher and more uniform level. Already at the beginning of the measurement series, 84-86% of drivers and front seat passengers wore a belt. Today, the levels are 92-94%.