DESIGN SPEED CHOICES FOR CANADIAN TWO-LANE RURAL HIGHWAYS

 Design speed is defined as “a speed selected as a basis to establish appropriate geometric design for a particular section of road” in the 1999 TAC Geometric Design Guide. While the TAC Design Guide has enhanced the various definitions of speed and placed an emphasis on the need for designers to recognize that operating speeds may be different from design speed assumptions, it does not provide specific guidance on how to choose an appropriate design speed. As part of TAC’s commitment to update the Geometric Design Guide on a regular basis two working papers have recently been completed to reflect new international developments in the areas of design consistency and design speed. This paper presents an overview of the salient findings from the working paper on design speed choices. The working paper is based on an analysis of design speed practices around the world. There are several differences in the design speed concept as applied to the design of rural alignment in Canada and other countries (except for the United States). Whereas the United States and Canada continue to adhere to the design speed concept as classically applied, many European countries and Australia have enhanced their use of design speed to incorporate explicit consideration of actual driver speed behaviour in terms of 85th percentile operating speed. Basically, design speed and operating speed research in a number of countries has found that, on curves with design speeds less than about 90 km/h, actual speeds are typically in excess of the design speed. Formal design procedures based on operating speed have been developed in Australia, France, Germany, Great Britain, Sweden and Switzerland. While the procedures differ, all place greater emphasis on the consistency of alignment standards rather than the traditional design speed approach. The TAC working paper recommends that an operating speed approach be adopted for rural two-lane highways in Canada. The recommended approach incorporates feedback loops in the alignment design procedures to identify and resolve operating speed inconsistencies. The proposed operating speed approach begins with selecting a nominal design speed; selecting design parameters for highway geometric elements; developing a trial alignment, and estimating the 85th percentile speeds on the trial alignment. A consistency check is then made to determine if the estimated speed matches the design speed. If the estimated speed does not match the design speed a new trial alignment is developed. If this is not possible another nominal (trial) design speed is selected and the process repeated until the estimated speed matches the design speed.