A study of speed zoning and traffic crashes in New South Wales : TARU research note 2/76

Speed zoning provides a means whereby speed limits may be tailored to suit the particular design and condition of and traffic stream characteristics on a given section of road. In 1965, several rural sections of highways in New South Wales were speed-zoned to replace the previously applying 50 mph prima facie speed limit (a prima facie speed limit permits a driver to exceed the stated limit, but if apprehended and prosecuted he must furnish proof that conditions at the time were such that it was in fact safe for him to do so). Since that time, speed zoning has been extended to other sections of road throughout the State. In all cases, speed zoning replaced the previously applying 50 mph prima facie speed limit. One of the well documented effects of imposing an absolute speed limit on a road is that the variance of the speed distribution of vehicles travelling along the road is reducedl. It is also well documented that increasing the spread of speeds in a traffic stream increases the crash potential of that traffic streaml. Thus, it is reasonable to expect that the introduction of rational speed zoning might have a beneficial effect on the traffic crash activity on the roads concerned. The purpose of this paper is to examine the traffic crash histories of speed zoned highways in New South Wales with a view to measuring the effectiveness of speed zoning in this State as a traffic crash countermeasure.