#22 Wide Centrelines

Wide centrelines on undivided 2-way roads are a Safe System supporting treatment to reduce the likelihood of head-on crashes between opposing traffic. The treatment consists of two parallel lines separating opposing lanes, spaced at a minimum of 0.5m and a maximum of 1.5m apart according to Austroads (specifications may vary by jurisdiction). The treatment is typically applied on rural roads where there is an identified risk of head-on crashes, but may also be applied in some urban settings.

Wide centrelines may be combined with audio-tactile line marking (ATLM) to alert drivers to imminent or partial lane departure and thereby improve the effectiveness of the treatment. Depending on the controlling road authority, provision for overtaking may be included on some sections. Wide centrelines may also be retrofitted with a median barrier such as wire rope, which then constitutes a primary safe system treatment (but precludes overtaking provisions).

Effectiveness

Wide centrelines are consistently associated with reductions in head-on and other crash types. The significance of these reductions varies according to a range of factors including treatment specifications, environmental characteristics and evaluation methods. The New Zealand Transport Agency cites 55% head-on crash reductions according to US research. However, a recent study which accounted for simultaneous increase in lane and shoulder width showed wide centrelines were associated with reductions of up to 74% for total injury crashes and 41% for head-on crashes. Where wide centreline treatment involved lane and shoulder width reduction, effects were less significant for total injury crashes (-44%) but greater for head-on crashes (-61%).

Considerations

As noted above, treatment effectiveness depends on a range of factors. If the existing overall road width is to be maintained, installing wide centrelines requires narrowing either the traffic lanes and/or road shoulders. The costs and benefits of this trade-off should be carefully assessed. While widening the cross-section to maintain or increase lane and shoulder width may be an ideal solution for adding wide centrelines, this will often not be feasible.