This report compares the speeds of cars and articulated trucks before and after the change in heavy vehicle speed limit to 100 km/h on 1 July 1988. Comparable data before and after this time were collected at 62 sites in New South Wales, Victoria, Queensland, South Australia, Western Australia and Tasmania. Faster articulated truck and car speeds were recorded at most (four fifths) of the sites after the speed limit change compared with before the change. The mean speeds of cars and articulated trucks were closer after the change compared with before for most sites. The percentages of sites for which truck speeds increased, for which car speeds increased, and for which car and truck speeds became closer rather than further apart, are each statistically significant. These three results remain true when the analysis is restricted to sites with appreciable data. They occur for both two lane and four lane roads, and for roads with car speed limits of both 100 km/h and 110 km/h. It is estimated that overall, cars have increased in free speed by about 1 km/h and trucks by nearly 2 km/h. The car truck speed difference is consequently estimated to have decreased by 0.7 km/h from 8 km/h to 7 km/h. This decrease is statistically significant. The variation in car speeds was higher after 1 July 1988 than before, although this was probably confined to roads with a speed limit of 110 km/h. The variation in truck speeds did not change significantly. The proportion of both cars and articulated trucks travelling faster than 110 km/h was higher after the speed limit change than before. Variation in car and truck density does not appear to affect car and truck speed changes although the change in the car truck speed difference is possibly inversely related to the change in the ratio of truck to car numbers (Author/publisher)
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