The safety of a horizontal curve (by means of accident frequency and degree of severity) is examined in this paper by features internal and external to it; both of these characteristics affect the driver attitude and the curve approach speed. Based on a review of the available literature on the subject, first the relationship between accident rate and some of the most important curve characteristics is examined. Then data from rural highways in Greece are used to investigate the relationship between accident rate and degree of curve or the curve radius. The flexible family of Gamma distributions is selected for this analysis and models using a special form of it (the shifted negative exponential distribution) are defined. The estimated functional forms are presented in conjunction with those from the literature and conclusions are drawn.
Horizontal alignment, horizontal curves safety, road geometric characteristics, rural roads
 Lamm, R., Psarianos, B. & Mailaender, T., Highway Design and Traffi c Safety Engineering Handbook, McGraw – Hill Handbooks: NY (USA), 1999.
 Raff, M.S., Interstate Highway-Accident Study, Highway Research Board Bulletin 74, Traffic Accident Studies: Washington D.C., pp. 18–45, 1953.
 Leisch, J.E. & Associates. Traffic control and roadway elements - their relationship to highway safety/revised. Chapter 12, Alignments. Highway Users federation for Safety and Mobility, 1971.
 Matthews, L.R. & Barnes, J.W., Relation between road environment and curve acci-dents. Proceedings, 14th ARRB Conference, Part 4, pp. 105–120, 1988.
 Deacon, J., Relationship between Accidents and Horizontal Curvature, Appendix D in Designing safer roads. Special Report 214, Transportation Research Board: Washington, D.C., 1986.
 Lamm, R., Choueiri, E.M. & Mailaender, T., Accident rates on curves as influenced by highway design elements - an international review and an in-depth study. Proceedings, Road safety in Europe, VTI Rapport 344A. VTI, Linkoping: Sweden, 1988.
 Zegeer, C.V., Stewart, R.J., Council, F.M., Reinfurt, D.W. & Hamilton, E. Safety Effects of Geometric Improvements on Horizontal Curves, Transportation Research Record 1356, Transportation Research Board: Washington D.C., 1992.
 Voigt, A., An Evaluation of Alternative Horizontal Curve Design Approaches for Rural Two-Lane Highways. Report No. TTI-04690-3, Texas Transport Institute: Dallas (USA), 1995.
 Anderson, I.B., Bauer, K.M., Harwood, D.W. & Fitzpatrick, K. Relationship to Safety of Geometric Design Consistency Measures for Rural Two-Lane Highways Transporta-tion Research Board of the National Academies 0361-1981 Volume 1658, 1999.
 Zhang, C. & Ivan, J.N., Effects of Geometric Characteristics on Head-On Crash Incidence on Two-Lane Roads in Connecticut Transportation Research Board of the National Academies 0361-1981 Volume 1908, 2005.
 Aram, A., Effective safety factors on horizontal curves of two-lane highways. Jour-nal of Applied Science, 10, pp. 2814–2822, 2010. doi: http://dx.doi.org/10.3923/jas. 2010.2814.2822
 Matsoukis, E. & Baladimas, D., Relating road safety and geometric characteristics at horizontal curves. The case of rural roads. Proceedings of the 3rd Panhellenic Confer-ence, Patras, Greece, October 2005.
 Baladimas, D., Safety Characteristics of Horizontal Curves, M.Sc. thesis, Department of Civil Engineering, University of Patras, Patras, Greece, 2009.