ROAD SAFETY ON HORIZONTAL CURVES: A LITERATURE REVIEW

Horizontal curves concentrate a disproportionate share of severe road crashes, with average crash rates about three times higher than on comparable tangent sections. This paper reviews the international literature on road safety at horizontal curves and synthesises three research streams that have developed largely in parallel: geometric design factors, speed and driver behaviour, and crash prediction methodology.
Fifty four sources, retrieved through a structured search of Scopus, Web of Science, ScienceDirect, TRID and Google Scholar, are organised by thematic strand and assessed for convergence of findings. Curve radius remains the dominant geometric predictor, while side friction demand, pavement condition, the relationship with adjacent curves and the combination with vertical alignment are established but underused
secondary factors. Methodological work has moved towards specifications that address excess zeros, unobserved heterogeneity and spatial structure, whereas evaluation of countermeasures remains the thinnest strand of evidence and rests on a small number of before and after studies. Five research gaps are identified: flat terrain settings, two part crash generating processes, maintenance variables, validation of behavioural surrogates, and the geographical narrowness of the evidence base.