Operating speed-profile prediction model for two-lane rural roads in the Italian context

The evaluation of changes in operating speeds along the alignment is effective in checking the consistency of the road because it can identify the horizontal curves that require an excessive speed reduction in respect of the approaching speed to be traveled safely. However, to use the operating speed-profile as a consistency tool it is necessary to accurately predict the speed as a function of the road geometry. Therefore, an operating speed-profile model was developed specifically for the Italian two-lane rural roads using speed data collected on tangents and curves representative of the common conditions found in Italy. The model includes prediction equations for desired speed on long tangents, operating speed on curves, acceleration and deceleration rates. The model takes into consideration the general character of the horizontal alignment, through the use of the curvature change rate of the road, to estimate the desired speed and to improve the accuracy of the speed prediction on curves. The operating speed-profile model proposed is an effective tool for the evaluation of design consistency and for other safety analyses of new and existing roads.