An essential part of the model for traffic simulation which is in preparation at the National Swedish Road Research Institute is a description of the variations in the speeds of road vehicles during their passage through horizontal curves. Studies of the speed adaptation on horizontal curves have been made at the Road Research Institute on several occasions in the course of the period from 1965 to 1967. A common feature of these studies is that they have been carried out under left-hand traffic conditions. It was therefore desirable to examine, among other things, the effect produced by righthand traffic on the speeds of vehicles on curves. It was to be expected that the sight from the inner lanes of the curves would be improved for vehicles equipped with steering wheels on the left-hand side, which are completely predominant in Sweden. In order to study the above-mentioned effect, investigations have been made in 1969 at some places where similar measurements had been performed in left-hand traffic, so that reference data were available for these places. In the first investigations, a study was made of free-moving vehicles. They were defined as vehicles separated by a time gap of at least 6 seconds from the vehicle ahead. The results of these investigations proved to be in close agreement with those of Taragin's tests carried out in 1954. It was found, among other things, that the speed adaptation took place in most cases before the curve, and was then maintained constant throughout the curve. Furthermore, the difference in speed between the vehicles moving in the inner and outer traffic lanes was very slight. In another investigation, the variation in the Speed of each vehicle with the distance was determined by measuring the travel time over 8 consecutive road sections. This investigation was carried out on free-moving vehicles at 25 places in Central Sweden, and covered 1 200 vehicles in all.