REDUCED SIGHT DISTANCE ON EXISTING RURAL ROADS. HOW CAN WE DEFEND IT?

This paper presents the results of research by the Queensland Department of Main Roads on sight distance issues as related to existing roads. The research was carried out in conjunction with the development of the Department's Road Planning and Design Manual. There are many sections of road in Queensland that have generally suitable horizontal geometry by virtue of the design practices or standards that were used when they were designed in the 1940's, 1950's and early 1960's. However, the vertical geometry often contains some crest vertical curves that are deficient in terms of current design standards: standards that have existed since the mid 1960's. At the same time, the vehicle speeds on many of these roads are significantly different from those assumed at the time of design. But, there is usually no accident history at the places that have deficient sight distance. Given the satisfactory operation of these roads, restoration has usually been favored over realignment in an era of restricted road funding. However, the design process must also ensure that the restoration does not change driver perceptions of the road to the extent that operation is compromised anywhere. Restoration is defined as works that improve the cross section and riding quality of a road but do not involve any change in the horizontal or vertical alignment. This situation is common to most developed countries. The effect of reduced sight distance on existing roads has been well researched overseas; in particular in the USA and Canada. This has been researched from as early as 1953 when it was found that drivers were overdriving crests that had been designed for the lower performance that vehicles had some 20 to 40 years earlier. Together, the USA and Canadian research indicates that there does not seem to be a problem as long as there is about 100m of sight distance and there are no intersections. The Canadian Design Manual is the only known 'primary road design manual' that formally recognizes this factor for restoration treatments for roads. In the USA, restoration is covered in a separate document to the primary AASHTO policy on geometric design. Overseas research to determine a link between accident rates and sight distance has been largely inconclusive. The Queensland Department of Main Roads formalized the use of reduced sight distance for crests on existing roads in 1981. As in the USA, this was done in a separate document to the primary design guide. This used a "reduced maneuver sight distance model". However, no explanation of the key maneuver time parameter was given. Nor was the operation of large commercial vehicles explicitly covered. Given the increasing need to be able to defend road geometry, this paper proposes how reduced sight distance may be defended in terms of the intent of the sight distance standards for a new road. This is because any case of reduced sight distance will inevitably be compared with the standard for a new road (in other words, what is done now). An extreme case of the reduced maneuver sight distance model is used as an example.

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  • English

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  • Accession Number: 00964335
  • Record Type: Publication
  • ISBN: 087659229X
  • Files: TRIS, ATRI
  • Created Date: Oct 9 2003 12:00AM