Effects of transition curves on sight distances

Maltinti F.

First

Investigation

;S. Portas

Second

Writing - Review & Editing

;M. Coni

Penultimate

Member of the Collaboration Group

;G. Piga

Last

Member of the Collaboration Group

2015-01-01

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Abstract

This paper presents an ongoing study on the effects of introducing transition curves into an existing road alignment on safety in terms of sight distances and design consistency conditions. Transition curves were firstly adopted in geometric design of railways in order to introduce circular curve naturally and the centrifugal force associated with it gradually in the alignment of transition sections. Later, they were also employed in geometric design of rural roads. In many Countries road geometric standards recommend their use to improve safe conditions. The main objective of the reported study was to evaluate the effect of transition curves on the respect of sight distances and design consistency conditions of a rural road with a high number of car accidents. Existing horizontal alignment of a rural road, with “tangent-to-curve” transition, was analyzed. Sight distances were calculated and verified with the imposed values by standards. Also speed gradients between contiguous elements were analyzed. A new alignment with transition curves and limited works out of the existing infrastructure, to keep the two alignments still comparable, was proposed. Then sight distances and speed variability along the proposed alignment were calculated and verified with the values imposed by the standards. The results obtained revealed that the introduction of transition curves reduces the percentage of verified sight distances along the horizontal alignment. However, different conclusions were obtained analysing the effectiveness of transition curves using design consistency criteria: the new alignment with transition curves have shown a more acceptable speed variation between contiguous elements.