Safety Index for evaluation of urban roundabouts.

Recently, there is a growing interest in road safety assessments based on the examination of the characteristics of the road aimed at identifying the presence of risk factors. This approach, named road assessment program or network wide road safety assessment, is required by the EU Directive 2019/1936 on road infrastructure safety management. Reliable procedures for assessing the inherent safety of all the elements of the road network are required to conduct roadway safety assessments. To provide a contribution toward the development of procedures for network wide road safety assessment, this paper develops and validates a Safety Index (SI) for evaluating urban roundabouts. The SI is assessed both at the roundabout level as well as at the roundabout approach level. This procedure detects the safety issues that are the largest contributors to crash risk in order to identify the safety measures that provide the greatest crash reduction at roundabouts. The SI is formulated by combining two components: the exposure of road users to road hazards (Exposure) and the risk factors which increase the probability of involvement in crashes (Risk Index). The procedure considers 33 detailed safety issues and 5 general safety issues to compute the Risk Index. Criteria for identifying and ranking the safety issues are defined. The SI procedure was validated in a sample of 50 urban roundabouts located in Rome, Italy. The sample consisted of 12 single-lane roundabouts and 38 two-lane roundabouts, with a total number of approaches equal to 179. In these roundabouts, the SI scores and the EB crash estimates were compared with reference both to the whole roundabouts as well as to the single roundabout approaches. The correlation between the SI scores and EB estimates was highly significant both at the roundabout level (R2 = 0.85, t = 16.49, p-value < 0.001) as well as at the approach level (R2 = 0.56, t = 14.88, p-value < 0.001). The results from Spearman's rank-correlation analysis provided further validation for the SI indicating that rankings from the SI and the EB estimates agree at the 99.9 % confidence level both at the roundabout level (ρs = 0.80) as well as at the approach level (ρs = 0.70).

Toward the development of a hybrid approach to speed estimation in urban and rural areas.

OBJECTIVE: Given the strong relationship between road accident and traffic speed, the evaluation and prediction of this latter have always been considered as a critical issue for road safety analysis and for the evaluation of road network safety improvements. Prediction models developed to date mainly focused on spot speed in a rural environment or on running speed in an urban one. Very few analyze the speed estimation in "transition" areas. The objective of this paper is to develop a generalized speed estimation model able to predict mean speed in urban, rural, and "transition" environment as a function of road layout characteristics. It is believed that the proposed estimation tool can be effectively employed by road engineers in the road safety design and retrofitting stage. METHODS: The basic idea of the paper is to shed some light on this issue by making use of a hybrid estimation approach able to combine the information gathered from both previously mentioned models within a generalized speed adaptation framework that reflects road user behavior. The calibration and validation of the generalized estimation model have been carried out following a collection of Floating Car Data (FCD) on several candidate sites. RESULTS: Preliminary results seem to indicate that the methodology proposed may be effective in estimating the spot speed in two-lane rural and urban arterials. CONCLUSIONS: FCD data can be useful to develop more efficient estimation models to better manage the safety of urban and rural roads.

An exploratory step for a general unified approach to labelling of road surface and tyre wet friction.

Wet skid-resistance is of paramount importance for road safety as it has been recognized to affect wet-road accidents. Recently European Regulations are introducing mandatory classification for tyre friction performance by means of tyre labelling procedure. In this paper an experimental study has been carried out in order to search for a relationship between indexes employed in the tyre and pavement classification. Coupled friction tests have been performed in a controlled manner on five test track with varying texture properties and significant statistical relationship has been derived between Wet Grip Index (WGI), as defined in the European Tyre Labelling Procedure and the International Friction Index (IFI), according to World Road Association (PIARC) friction harmonization experiment. If a temperature correction of skid data is applied a good correlation between WGI and IFI can be obtained. Although the experimental study has to be integrated with a wider measurement campaign, preliminary results seem to indicate that a unified wet tyre-road classification can be pursued allowing a better awareness of the road safety level among road users.