Åse Svensson

Evaluation of traffic safety, based on micro-level behavioural data: Theoretical framework and first implementation

A traffic encounter between individual road users is a process of continuous interplay over time and space and may be seen as an elementary event with the potential to develop into an accident. This paper proposes a framework for organising all traffic encounters into a severity hierarchy based on some operational severity measure. A severity hierarchy provides a description of the safety situation and trade-off between safety and efficiency in the traffic system. As a first approach to study the encounter process, a set of indicators is proposed to describe an encounter. These indicators allow for a continuous description even if the relationship between the road users changes during the process (e.g., when they are on a collision course or leave it). Automated video analysis is suggested as a tool that will allow data collection for validation of the proposed theories.

Cyclists in roundabouts -- Different design solutions.

Whether the safest roundabout design for cyclists is to separate cycle crossings or integrate cyclists with motorists is an extensively discussed issue. Studies using accident statistics indicate that a separated cycle crossing is the safest for high motor vehicle volumes. However, the results have not been satisfyingly explained. This article combines quantitative and qualitative methods in traffic conflict, interaction and behavioural studies to find out how interactions and conflicts differ between the two roundabout designs. Automated video detection is used as one of the methods and its performance is evaluated. The integrated roundabout turns out to be more complex with a higher number of serious conflicts and interaction types. The most dangerous situations in the integrated roundabout seem to come about when a motorist enters the roundabout while a cyclist is circulating and when they are both circulating in parallel and the motorist exits. The yielding rules are more ambiguous in the separated roundabout, contributing to a lower yielding rate to cyclists and a lower trust in the other road users willingness to yield. Situations in the separated roundabout with the lowest yielding rate to cyclists occur when the motorist exits the roundabout at the same time as cyclists are riding in the circulating direction and hence coming from the right. However, most of the accidents in separated roundabouts occur while cyclists are riding against the circulating direction, both when motorists enter and exit the roundabouts.

In search of the severity dimension of traffic events: Extended Delta-V as a traffic conflict indicator

Most existing traffic conflict indicators do not sufficiently take into account the severity of the injuries resulting from a collision had it occurred. Thus far, most of the indicators that have been developed express the severity of a traffic encounter as their proximity to a collision in terms of time or space. This paper presents the theoretical framework and the first implementation of Extended Delta-V as a measure of traffic conflict severity in site-based observations. It is derived from the concept of Delta-V as it is applied in crash reconstructions, which refers to the change of velocity experienced by a road user during a crash. The concept of Delta-V is recognised as an important predictor of crash outcome severity. The paper explains how the measure is operationalised within the context of traffic conflict observations. The Extended Delta-V traffic conflict measure integrates the proximity to a crash as well as the outcome severity in the event a crash would have taken place, which are both important dimensions in defining the severity of a traffic event. The results from a case study are presented in which a number of traffic conflict indicators are calculated for interactions between left turning vehicles and vehicles driving straight through a signalised intersection. The results suggest that the Extended Delta-V indicator seems to perform well at selecting the most severe traffic events. The paper discusses how the indicator overcomes a number of limitations of traditional measures of conflict severity. While this is a promising first step towards operationalising an improved measure of traffic conflict severity, additional research is needed to further develop and validate the indicator.

Pedestrian planning and the challenges of instrumental rationality in transport planning: emerging strategies in three Swedish municipalities

Abstract Despite having been subject to a strong and many-sided critique, instrumental rationality persists as paramount in transport planning practice and research. This paper argues that it is crucial to acknowledge the prescriptive and normative facet of instrumental rationality in order to understand its impact on transport planning. The aim is to explore to what extent pedestrian planning – within a specific geographical context – has subscribed to the methods employed within the limits of instrumental rationality. The study draws on data from interviews with urban and transport planners in three municipalities in the southern part of Sweden (Malmö, Lund, and Helsingborg) together with those municipalities’ urban and transport planning strategies. The material is analysed through three aspects related to (1) who the road user is in the realm of instrumental rationality, (2) what kind of knowledge is of interest within instrumental rationality and (3) how instrumental rationality takes physical form. The analysed material displays walking as increasingly being included in planning exercises and general design guidelines. However, these efforts do not form a consistent and thought-out strategy proceeding from instrumental rationality. Walking remains seemingly excluded from many methods defining this concept. Furthermore, the means are not described as explicitly related to certain ends, or vice versa.