Preliminary communicationThis paper presents models for predicting operating speeds on tangent sections and curves of two-lane rural roads developed on continuous speed data. The data corresponds to 20 drivers of different ages and driving experiences, driving their own cars along an 18 km long section of a state road. The data were first used for determination of minimum operating speeds on curves and maximum operating speeds on tangents and their comparison with speeds in the middle of curves and tangents i.e. speed data used in most of the operating speed studies. There was no significant difference between speed models developed using speed data in the middle of tangents and curves and models developed using maximum and minimum operating speeds on tangents and curves. Models developed on continuous data have higher coefficient of determination than models developed on spot speed data. It can be concluded that the method of measuring has more significant impact on the quality of speed model than the location of measurement.Keywords: operating speed; continuous speed data; tangent sections; curve; spot speed; consistency Modeli operativne brzine vangradskih dvotračnih cesta temeljeni na kontinuiranim podacimaPrethodno priopćenje Ovaj rad prikazuje modele za predviđanje operativne brzine na pravcima i krivinama dvotračnih vangradskih cesta razvijene na kontinuiranim podacima o brzinama vožnje. Podaci se odnose na 20 vozača koji su vozili svoja vozila duž 18 km duge dionice dvotračne državne ceste. Podaci su prvo korišteni za određivanje minimalne operativne brzine u krivinama i maksimalne operativne brzine na pravcima te njihovu usporedbu s brzinama, u sredini krivina i pravaca, koje se najčešće koriste za razvoj modela operativnih brzina. Nema značajne razlike između modela brzina razvijenih na temelju podataka o brzinama na sredini pravaca i krivina i modela razvijenih na temelju podataka o maksimalnim i minimalnim operativnim brzinama na pravcima, odnosno krivinama. Modeli razvijeni na temelju kontinuiranih podataka imaju veći koeficijent determinacije od modela razvijenih na temelju točkastih podataka o brzinama u sredini pravaca i krivina. Stoga se može zaključiti da metoda mjerenja ima značajniji utjecaj na kvalitetu modela brzina od mjesta mjerenja.Ključne riječi: operativne brzine; kontinuirani podaci o brzinama; pravac; krivina; brzina u presjeku; konzistencija

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Experimental investigation was conducted on a 24 km long segment of the two-lane state road to collect the driver behavior data. The research involved 20 drivers driving their own cars equipped with the GPS device. Considering the impact of path radius and speed on the side friction demand, the design consistency on horizontal curves was evaluated by determining the margins of safety. The analysis showed that the vehicle path radii were mainly smaller than curve radius, on average for 12%. Regression analysis indicated that the percentage difference between the curve radius and vehicle path radius is not affected by the speed, speed differential and geometric characteristics of the curve and surrounding elements. Two different margins of safety were analyzed. One is the difference between maximum permissible side friction (based on design speed) and side friction demand, while another is the difference between side friction supply (based on operating speed) and side friction demand. Generally, demands exceeded supply side friction factors on curves with radii smaller than 150 m, whereas “poor” conditions (in terms of Lamm’s consistency levels) were noted for curves under approximately 220 m. Both values are very close to the critical radius below which higher accident rates were observed according to several accident studies. Based on the results of the research, it is proposed to use a 12% smaller curve radius for the evaluation of margin of safety and that curves with radii smaller than 200 m should be avoided on two-lane state roads outside the built-up area.

Design of curves and their adjacent elements presents the greatest safety problem on rural two-lane roads. The use of the existing alignment consistency safety criteria (design, operating speed, and driving dynamic consistency) could have some shortcomings, especially in countries where the project or design speed is in use instead of (higher) operating speed. The consequence is that the designer should use smaller cross fall on curves than needed, while the calculated side friction is lower than in reality. Further, the existing graphs of adjacent curve radii do not take into account that there is a maximum operating speed achieved for a certain radius or long tangent above which it does not increase. This paper presents a methodology for determination of adjacent horizontal curve radii, with and without tangent between, based on the operating speed models which include dependence of operating speeds on tangents and curves on speed of adjacent alignment elements as well as maximum tangent and curve speed. The developed graphs of adjacent radii at the same time include the limiting values of driving dynamic consistency criteria, so the road designer does not need to calculate permissible and demand side friction for every combination of adjacent alignment elements.

This research analysed the impact of road alignment on the fuel consumption and gas emissions of a vehicle driven at a free-flow speed by an 85th percentile driver. The field experiment included constant and free-flow speed rides by a personal car equipped with a high-performance (10 Hz) Bluetooth global position device and on-board diagnostics connector, with which the travel path, speed, acceleration, and consumption data were recorded. Regression analyses of the dependence of free-flow speed and fuel consumption on the geometric characteristics of the road (curvature, length, longitudinal slope, etc.) resulted in the formation of a reliable model that could compare alternative road designs for a given corridor in terms of route economy and safety. The main parameters contributing to the consumption on tangents were the slope and radii of adjacent curves. For constant speed, the slope was the only geometric parameter that had an impact on fuel consumption.