A Method to Manage Design Solutions Outside the Road Standard Criteria

This paper proposes a methodology that could prove to be useful in the geometric composition of roadways when any variables involved in the problem exceed the limits imposed by the norms. In fact, if the rigid application of standards leads to unattainable configurations or, at least, those not fully respecting the economic and environmental restrictions, it may be necessary to verify the actual safety of those roadways only marginally outside the norm threshold, with appropriate mathematical instruments. This flexibility, therefore, must never be gained at the cost of the safety of drivers. Achieving greater flexibility in standards may well lead to a decrease in safety for drivers. What designers need to ensure is that any decrease in safety is only marginal and that the level of safety provided is appropriate for the circumstances. Although much progress has been made in the definition of design solutions sensitive to the site context, nevertheless the final choice is still based on the checklist completion and on the judgement of expert analysts, without resorting to rigorous analytic methodologies. Proceeding this way, though easily applicable, is too subjective and strongly dependent on the experience which cannot always be guaranteed. To prevent such points, a methodology has been prepared, in this article, which permits the evaluation of the safety level, in terms of risk, of all the possible design solutions. This instrument, with few calculation operations carried out with the Interval Analysis technique, is able to make the best choice among those available, by altering the input variables involved in the physical phenomenon. The final choice could therefore fall back onto a solution of which one or more variables exceed the thresholds allowed by the norm. In this way, the designer could ask for a simple exception, motivated by a specific rational analysis. The overall process was based on the One Step Below criteria from the British Standard TD9/93. The procedure is applied to the study of a transition curve and requires the provision of general-use tables and graphics, which, if extended to all the geometric elements, would render it usable by lessexpert users and eliminate the need for further analytic operations.