Review of the development of US roadside design standards.

A review of the development of US roadside design standards was undertaken in the context of understanding the marked differences between North American and European roadside design practices. The US standards are controlled by a policy of providing a 9 metre clear zone on relatively flat roadsides, or equivalent, within which errant vehicles can recover. Research into the effectiveness of the clear zone policy found definite safety benefits. However, the roadside hazard model, which was developed to guide the implementation and retrofitting of the policy , was found to greatly overestimate the magnitude of these benefits. It is shown that there were deficiencies in the data set used to derive the model, which would have contributed to this discrepancy. The original roadside hazard model predicted limited safety benefit for clear zone widths less than about 8 metres. More recent research has shown a diminishing return between safety benefits and increasing clear zone width, with more than 85 percent of benefits from a 9 metre clear zone being captured in the first 6 metres. This implies potential for reasonable safety-cost tradeoffs where it is difficult to provide ful1 51101A.