Clear zones, barriers and driving lines – mitigating the effects of crashes on corners (horizontal curves)
Research was undertaken to further the development of practices and guidelines for the use of clear zones and barriers on corners by investigating the origin and extent of run-off-road vehicle encroachments. Under the “safe system” approach to road safety, the design and use of clear zones and barriers is being re-examined. This is to reflect the desire for safer roadsides, as well as current geometric design practices, skid resistance levels, vehicle performance, and driver behaviour. This paper presents the findings of a study that investigated the drive lines taken by drivers through a range of corners with radii less than 250m radius. The drive lines identified were then applied to computer simulation modelling using the software package PC Crash, together with corner models generated from the available road geometry data. Vehicle types, speeds, road conditions and roadside conditions were varied to identify the origin and extent of encroachment out of the sealed lane. Limited modelling was also carried out to assess the effects of increasing the width of the sealed shoulder, and changing the roadside slope. Some of the principal findings of this study were that, (1) variations between different vehicles in the origin and extent of encroachment were relatively small for similar speeds and driving lines, (2) encroachments in dry conditions were also relatively small, (3) vehicles encroaching in wet conditions can pass through a standard clear zone width of 9m, reaching the far side with relatively high speeds, even under emergency braking, (4) the geometry and friction of the roadside have a significant effect on encroachment distances, and (5) seal width extensions of 1-2m can significantly reduce encroachment distances.