Metric analysis

New metric for finding the scariest intersections in a city

When city officials and city planners measure street safety, they usually do so by counting the number of crashes: how many times a driver has hit another driver or hit a pedestrian crossing the road at that intersection particular? AT Megan Ryerson, a transportation engineer and urban planner at the University of Pennsylvania, this view of safety is too narrow. We can count accidents and deaths, but those numbers tell us nothing about how people to feel when moving around a city.

This metric also limits how we even begin to measure the concept of “security”. A crash hotspot map doesn’t tell us which routes people avoid entirely because they don’t feel safe on that road. It also means that a safety measure can only be put in place after a certain number of accidents, so there is a “literal human cost” to how we measure or plan for safety, Ryerson says. Instead, she created another metric, based on biometric data that can identify dangerous or difficult parts of urban infrastructure before an accident occurs.

For his study, a group of cyclists cycled around Philadelphia wearing eye-tracking goggles and a gyroscope that collected data on when and where they moved their eyes, and how often and how often. moments of their journey they turned their heads. These movements indicated the cognitive workload and stress levels experienced by cyclists when traversing different types of road infrastructure, such as riding on a protected cycle path for a section, then in a “mixing zone” – an area where there is no physical separation between bikes and cars – the next.

The researchers tracked four indicators of workload: gaze speed (the amount of eye movement per second) and position; vertical and lateral angles; and vertical and sideways gyroscope angles, which correspond to whether riders turn their heads to check over their shoulder or tilt their heads up and down to look for potholes or read road signs.

“When the cognitive workload is very high, the risk that you will make a mistake is much, much greater than when your cognitive workload is low,” says Ryerson, also lead author of the study, published recently in the review Accident analysis and prevention. If a road design is already causing a high cognitive workload and something unexpected happens – a pedestrian coming off the sidewalk or a driver veering into the bike lane – it becomes more difficult for a cyclist to process this new information and react in time.

Measurement of cognitive workload based on eye and head measurements is not new. It is used to set policies on how many planes an air traffic controller can handle at once, and speed limits around curves, based on how far the driver can see ahead. For anyone who has cycled in a city, it’s obvious that stretches of road increase your stress levels and require more concentration, compared to areas like a protected bike path, where there’s relatively little fear of an accident. Ryerson’s findings based on biometric data provide concrete evidence behind these sentiments.

In this way, his findings are not so surprising. “I’m not the first person to say that mixing zones are less safe [than protected bike lanes], and we even know from accident data that mixing zones are where the most accidents happen compared to protected zones,” she says. “But what I can say now is, hey, I have four new variables that are continuous and human-focused, that can be used to measure safety rather than waiting for someone to die. “

While the fact that cyclists experienced higher stress and cognitive workload in the less protected areas of the road was not shocking, there was a startling finding: regardless of their experience level, every cyclist has experienced their highest stress moments in the same places. The researchers found this by normalizing an individual’s stress and workload levels to their own baseline, rather than looking at all 39 participants and extracting the highest measures. “Even if, for example, I am a ‘nervous cyclist who looks everywhere’ and you are a ‘confident cyclist who looks straight ahead’, we both experienced our highest stressful moments in exactly the same spaces” , says Ryerson. . “This finding alone indicates that it is the infrastructure that drives these responses.”

Now that Ryerson has established these new metrics to measure safety, it’s using them to do a before-and-after comparison of a recently installed bike lane in Philadelphia, to see if the change in infrastructure has had an effect on rider stress. and cognitive workload. She hopes it will help change the definition of accident and fatality safety to be about stress, workload and how pedestrians and cyclists feel. “By doing so,” she says, “I think we will have a more holistic view, a more human-centric view, of safety in our cities.”