This paper describes the development and use of an Extended Design Domain (EDD) for road restoration projects. It builds upon material in four of the papers presented at the Second International Symposium on Highway Geometric Design plus other recent research. Over the last 25 years, most Australian road authorities have found that restoration projects form a significant amount of their project work. Restoration projects are ones that primarily keep the existing road alignment and just improve the cross section. Commonly, the vertical alignment retains crest vertical curves that are deficient in terms of current design requirements for the operating speeds. Similarly, some intersections have less sight distance than what would be provided for new roads. At the same time, there is usually no accident history at these places. As a result, restoration projects have posed difficulties for designers that have bordered on the ethical. Designers have been forced to accept the situation without any plausible guidelines on what sight distance was acceptable and why. The EDD is a range of design values below the minimums currently used in Australian road design guidelines. The EDD is applicable for assessing the geometry of existing roads and upgrading the geometry of small sections of existing road. The new values are based on sound engineering grounds and use data from modern and comprehensive international and Australian research. This gives justification to the use of these values, which is important in the event of litigation. The geometry of significant lengths of existing road can often be retained because EDD shows that these roads have reasonable sight distance and cross section capability for the future traffic operation. This is turn shows that restoration projects are fit for purpose or context sensitive solutions because engineering benefits can be identified along with the economic benefits.
Abstract