An experimental investigation on the relationship between speed and road geometry

In this work we had to face the topic of intrinsic road safety experimentally by investigating two interurban roads. These roads are characterised by different types of traffic. The results of the investigation are given in the form of prediction relations with their relevant speed diagrams. The present investigation, to be carried out in the field, intends to verify, on some sections considered as significant, the relationship and mutual incidence between the variables flow, road geometry and speed. In the processing of the results, use will also be made of specific instruments for evaluating the geometrical consistency of the road alignment in accordance with operating speed. Our experimental investigation aimed at studying operating speeds on two different categories of roads as regards both importance and circulation. First of all we selected the road samples which belong to the A18 Messina-Catania motorway and to the SS 626 Licata-Ravanusa state highway. The study contemplated the use of a laser beam speedometer and some digital camcorders. In each section we considered two cross-sections at the same time, one on a tangent and one on a curve. At the same time, we also measured traffic flows which were subsequently disaggregated according to all the calculation parameters necessary to establish the level of service. In order to have a clearer interpretation of the experimental evidence and to propose some prediction relations we analysed the rapport between V85 and some geometrical parameters like CCRs, CCR, is and i. Both prediction relations and their determination coefficients show that it is the simultaneous and synergic action of planimetric and altimetric elements that determines a high degree of correlation to V85. In both road samples we found that operating speed values are almost always higher than the design speed of the geometrical elements considered. On the other hand, a general increase in operating speed involves very small speed variations in the transition from one element to the other. This leads us to think that the layouts are appreciably consistent, as can be seen from the diagrams prepared on the basis of the prediction reports V85-CCRs-is. As for the motorway sections studied, this datum conflicts with what the curvature planimetric-design speed diagrams, prepared on the basis of the present Italian geometric design standard, affirm. We think that these prediction relations could become behavioural models, on the basis of more thorough studies, referring to a specific category of road geometry characterised by a specific mobility function. Moreover, the values of the evaluation parameters of the consistency of the road should vary – although the intervals should always be small – in relation to both the geometrical and functional characteristics of the road.