#10 Clear Zone

Clear zones

A clear zone is an unobstructed area adjacent to a traffic lane which allows an errant vehicle to stop safely or to regain control. Removal of roadside hazards to provide clear zones is an effective measure to reduce the impact and severity of lane departures and run-off-road (ROR) events. In a safe system context, clear zones are considered by Austroads as a supporting treatment only, as some considerable risks remain with this approach.

Effectiveness

Effectiveness of clear zones depends on numerous variables, including the clear zone width, angle of vehicle departures, road speed and horizontal curvature, and the nature of the roadside area. Generally, benefits increase with the distance between the traffic lane and roadside features.

In Australia, compared with no clear zone, clear zone widths of 4m and 8m were found to produce 49% and 54% crash reductions respectively (Austroads 2024, Guide to Road Safety Part 2).

In the US, increasing the distance to roadside features from 1m to 5m produced a 22% crash reduction, while an increase from 5m to 9m reduced crashes by 44% (US Federal Highways Administration).

Considerations

Establishment of clear zones is frequently precluded by a range of factors, such that appropriate barrier systems will often provide a safer and more cost-effective alternative.

Clear zones are associated with rollover crashes, the likelihood of which increases with poorly compacted roadside surface and higher angle vehicle departures. This contributes to the higher severity of clear zone crashes when compared with continuous flexible barrier treatments.

The effectiveness of clear zones may vary depending on the vehicle type. For example, motorcycle lane departures may be less severe on clear zones compared with where barriers are installed, as barrier posts contribute to rider injury.