Heavy vehicle safe cornering speeds.

The prediction of safe cornering speed based on the formulae presented in the Austroads publication 'Rural Road Design', in general, crudely assumes heavy vehicles pass through the corner and subsequently pass the corner's apex at constant lane side clearance. This paper examines various relaxations to this assumption. Firstly, the author examines the predicted safe cornering speed assuming a heavy vehicle simply operates in the lane space between the corner initial and final boundary conditions, and finds that typical heavy vehicles can safety negotiate corners some five to eight km/h above that predicted by the crude model. The second relaxation is to assume the heavy vehicle passes through the corner along the 'smoothest' path. Notably sufficiently accurate mathematically valid simple graphical long hand curve fitting procedures can be conveniently employed. Even here it is highlighted that the prediction accuracy is very dependent on knowledge of the actual curve geometry, commencement and final inflexion points and transitory curve details. Taking into account pavement super-elevation and general upgrade and downgrade details, cornering speeds consistent with that adopted by experienced professional heavy vehicle drivers operating along familiar roads are predicted. This advanced cornering analysis eliminates the confusion that heavy vehicles should be falling over on every advisory speed signed corner. The same highlights the importance of driver skill and the need for previous corner knowledge (familiarity with the road). The contrary of the latter suggests the need for improved signage or that heavy vehicle drivers slow down whenever operating on 'non familiar' roads. The need for the latter is particularly relevant should the approach to unfamiliar corners occur under night time operating conditions. Certainly subject to such conditions the best advice is for heavy vehicle drivers to 'slow down' whenever operating along unfamiliar roads and more so when operating night time. (Author/publisher) For the covering entry of this conference, please see ITRD abstract no. E205861.