The cost-effectiveness of curve flattening in Alberta

Abstract

Rehabilitation and restoration improvements on highways typically involve widening lane and shoulder widths, as well as revisions to the superelevation rate. This upgrading approach may sometimes neglect the possibility of flattening horizontal curves, which may result in safer, more cost-effective designs. Aside from the obvious safety benefits for highway travellers, additional benefits may occur in terms of improved traffic operations of intersections on curves. This paper describes the findings of a research project which was conducted to determine the maximum side friction available for a range of roadway curvatures, vehicle speeds, and vehicle types.A three-axis accelerometer and a ball bank indicator were installed in seven test vehicles. Lateral accelerations and ball bank readings were recorded as test vehicles traversed test curves at constant speeds. These data were used to determine the margin of safety provided on dry, wet, and icy pavements.It was found that motorists demand high levels of side friction on sharp curves. In fact, curvatures sharper than 500 m provide very low margins of safety against skidding. Since improved superelevation rates are unlikely to increase the margin of safety to a sufficient level, the cost-effectiveness of curve flattening for rural highways in Alberta was estimated for a range of traffic volumes and central angles. Key words: side friction, margins of safety, horizontal curves, 3-R projects, cost-effectiveness of curve flattening.