Large-Scale Microscopic Traffic Behaviour and Safety Analysis of Québec Roundabout Design
Roundabouts are a relatively new design for intersection traffic management in North America. With great promises from abroad in terms of safety, as well as capacity—roundabouts are a staple of European road design—roundabouts have only recently proliferated in parts of North America, including the province of Québec. However, questions still remain regarding the feasibility of introducing the roundabout to regions where driving culture and road design philosophy differ and where drivers are not habituated to their use. This aspect of road user behaviour integration is crucial for their implementation, for roundabouts manage traffic conflicts passively. In roundabouts, road user interactions and driving conflicts are handled entirely by way of driving etiquette between road users: lane merging, right-of-way, yielding behaviour, and eye contact in the case of vulnerable road users are all at play for successful passage negotiation at a roundabout. This is in contrast with typical North American intersections managed by computer-controlled traffic-light controllers (or on occasion police officers) and traffic circles (Wallwork 1991) of all kinds which are also signalized. And while roundabouts share much in common with 4 and 2-way stops, they are frequently used for high-capacity, even high-speed, intersections where 4 and 2-way stops would normally not be justified. Resistance to adoption in some areas is still important, notably on the part of vulnerable road users such as pedestrians and cyclists (Hydén 2000, Stone 2002, Granà 2011, Perdomo 2014) but also by some drivers too.
While a number of European studies cite reductions in accident probability and accident severity, particularly for the Netherlands (Schoon 1994), Denmark (Jensen 2013), and Sweden (Bergh 1997, Hydén 2000), research on roundabouts in North America is still limited, and even fewer attempts at microscopic behaviour analysis exist anywhere in the world. The latter is important because it provides insight over the inner mechanics of driving behaviour which might be key to tailoring roundabout design for regional adoption and implementation efforts.
Fortunately, more systematic and data-rich analysis techniques are being made available today. This paper proposes the application of a novel, video-based, semi-automated trajectory analysis approach for large-scale microscopic behavioural analysis of 20 of 100 available roundabouts in Québec, investigating 37 different roundabout weaving zones. The objectives of this paper are to explore the impact of Québec roundabout design characteristics, their geometry and built environment on driver behaviour and safety through microscopic, video-based trajectory analysis. Driver behaviour is characterized by merging speed and time-to-collision (Laureshyn 2010), a maturing indicator of surrogate safety and behaviour analysis in the field of transportation safety. In addition, this work represents one of the largest applications of surrogate safety analysis to date.
A number of roundabout safety studies have been performed throughout Europe. In the Netherlands, for example, one study found a decrease in casualty rate across 46 roundabout conversions of up to 74% (Schoon 1994) (though admittedly the rates were small to begin with). A more recent example in Denmark shows important reductions in accident rates and accident severity across a large data set, though it also suggests that roundabouts have the least effect on mitigating property-damage-only (PDO) collisions (Jensen 2013). This study also looked at contributing factors and recommends tapered central islands above installations with islands which are not elevated or that have elevations with cylindrical shape (obstruction of visibility). Finally, it notes that cyclist collisions increased over the same period.
Experience and research in North America are still lacking, though some efforts have nevertheless been made. One study found a decrease in collision severity, particularly for fatal collisions, using an empirical Bayes model on 24 stop-controlled intersection conversions into roundabouts (Persaud 2001, Rodegerdts 2007). A more recent, but similar, study found essentially the same result across 28 sites in the same region (Gross 2013). Meanwhile, closer to Québec, Burns found that large passenger vehicles, multiple vehicles, and night time were associated with increased accident severity (Burns 2013).
Surrogate safety analysis is a pro-active road safety diagnosis methodology which aims to improve road safety analysis methods by complementing historical accident data (or supplanting it altogether when it is not available) with cheap and short observations of ordinary traffic behaviour (Tarko 2009). Speed is a classic surrogate safety measure, though this designation is rather new: many studies in the literature infer from or target speed directly for purposes of road safety. Its effects on collision severity are well known, though its effects on collision probability are less sure (Hauer 2009).
Roundabout speed is consistently measured around 30 km/h in the literature (Chen 2013). In fact, it has been observed that, while high-speed areas typically have their speed decrease to 30 km/h after implementation of a roundabout, areas with lower speeds (e.g. 20 km/h) can have their speed increased to 30 km/h as well (Hydén 2000). This effect has been also observed at the microscopic level in Québec (St-Aubin 2013).
There are many other surrogate safety measures, but time-to-collision (TTC) is the surrogate measure of safety of choice for its relative maturity, simplicity, and transferability properties. TTC measures the time remaining, at any instant in time, before two road users on a potential collision course collide: higher values are better for safety. It does not have the same level of validation in the literature as speed, but while speed is a good predictor of collision severity, TTC promises to be a good predictor of collision probability, a property which is arguably lacking with speed (Hauer 2009). Therefore, modelling both speed and time-to-collision should give a good overall representation of collision risk associated with road user behaviour.
Several collision-course modelling techniques are used in the literature, chief amongst them in terms of ubiquity is constant velocity (Laureshyn 2010). However, the constant velocity motion-prediction model is deemed inadequate for TTC measurement in roundabouts, as road users in roundabouts rarely follow straight trajectories, both inside the roundabout and on a significant portion of the approach. Fortunately, some more sophisticated naturalistic motion-prediction models have been developed to overcome this shortcoming: motion patterns are used for their ability to learn normal movement within a traffic scene. A discretized motion-pattern matrix method has been developed specifically for roundabouts (St-Aubin 2014).