Detection and low-cost engineering improvement of inconsistent horizontal curves in rural roads.

The topic of safety on horizontal curves is not restricted to the issue of geometric consistency. Visibility (how easy it is to perceive that there is a curve ahead and at what distance) and readability (how easy it is for a driver to evaluate the curve geometry, in order to adopt an adequate behaviour) are two additional characteristics with significant impact in safety at curves. Several measures can be applied to improve the visibility and the readability of road curves, such as road marking (including central no-passing areas), markers and landscaping (for example, low bush on the inside of the curve and high scrub on the outside). Pavement management systems should pay special attention to reductions in friction coefficients and macrotexture on curves to prevent increases in accident risk due to low skid resistance. Due to a higher probability of vehicles leaving the road on curves, requirements concerning obstacle free zones on the roadside of curves should be more severe than on tangents. Trial tests on a Portuguese main road (IP 5) have shown that the application of the signing system for curves presented in Chapter 3.3 has improved quickly and significantly some important aspects of driver behaviour: lower unimpeded speeds (-5-11 km/h, in the 85th percentile unimpeded speed on curves); better compliance of cars and trucks to the defined lane space; less vehicle paths over the inside shoulder (from 2050 per cent to 535 per cent); larger distances to the right edge line for vehicles in the inside lane (+25 cm). Overall, driver behaviour after the improvement in road signing is more homogeneous both in curves and in tangents, than in the before improvement period. It is expected that the widespread application of the method for the detection of inconsistent horizontal curves in single carriageway rural roads of the National Road Network, and the improvement of their signing and geometric characteristics (scheduled to start soon at another main road), will contribute to a significant improvement in the Portuguese road safety records. (A) For the covering abstract of the conference see ITRD no 207828. The reprints are also available at the web - http://www.vti.se/pdf/reports/K18APart1.pdf; http://www.vti.se/pdf/reports/K18APart2.pdf and http://www.vti.se/pdf/reports/K18APart3.pdf.