Australian Experience with Using Speed Prediction Models to Determine Accurate Design Speeds

Design Speed is one of the most fundamental parameters used in the geometric design of roads. It sets the general standard of the road and the appropriate design domain (that is, range of values) for many other geometric design parameters. Historically, a single design speed has usually been assumed for a road project taking into account the physical constraints on the road alignment together with some recognition of the standard of any adjoining sections of the road. Furthermore, less important roads have commonly been designed to a lower standard by simply assuming a lower design speed. Yet it is a fundamental requirement for designing safe roads to ensure that the road geometry suits the way the road will be driven. This may seem obvious but drivers are not aware of the design speed of the road. Drivers are largely influenced by their perception of the general standard of the road, horizontal curvature, cross section, signage, speed limits and the road environment in how they drive the road at any point. In 1980, the national road design guide for Australia set the 85th percentile operating speed as the basis for design speed. It also introduced the Speed Environment model for predicting 85th percentile speeds on rural roads based on Australian research. On less important roads, operating speeds were found to vary due to drivers speeding up on straights (usually above the design speed) and slowing down for curves that in some cases were still driven at higher than the design speed. However, the uptake of the original Speed Environment model was poor and even the uptake of its successor, the Operating Speed Model, has been slow. Many designers have continued to prefer the simpler practice of assuming a design speed. This paper therefore covers the following aspects of design and operating speeds: (1) The benefits of using a speed prediction model to determine design speeds and the surprising level of accuracy of the Australian models when properly used. (2) The extra design effort required to use the Operating Speed Model. (3) The importance and, at times, difficulty in designing a road that that matches the way it is driven. (4) The implication of potential tort liability if an assumed design speed does not match how the road is driven.