The Importance of Outliers
On average, car crashes cause more than 40,000 deaths per year in the United States. Technologies like seat belts, advanced airbags, and automated braking systems have improved car driver and passenger safety, but pedestrian deaths due to crashes have actually increased by 48% over the last decade, reaching about 7,500 fatalities in 2022.
Transportation researchers comb through police crash reports to identify infrastructure countermeasures that will help in the greatest number of cases. However, sometimes improving the average situation isn’t enough.
“By using traditional analysis methods that focus on the average, most studies on pedestrian safety overlook crashes that are rarer but may cause disproportionately high-risk injuries,” said Zeinab Bayati, a PhD student in the Department of Civil and Environmental Engineering (CEE).
The most common pedestrian crashes occur during the day and at intersections, but such crashes also usually result in comparatively minor injuries. High-risk injury scenarios—those in which pedestrians are much more likely to be seriously injured or killed—are less common but still vital to consider, said Bayati’s doctoral advisor, CEE Beaman Professor Asad Khattak.
“Let’s say it’s nighttime, there’s torrential rain, and maybe the pedestrian and the person who is driving are both under the influence of alcohol,” said Khattak. “Those kinds of situations result in more dangerous crashes, but they are so far outside the average that researchers might even remove them from the data set as outliers to make the general trends clearer.”
Using AI methods, Bayati and Khattak developed a novel framework that analyzes pedestrian crash data and sorts events into meaningful groups. Their research, published in the journal Accident Analysis and Prevention this January, revealed that safety measures aimed at the most common crashes might not save the most lives.
“We identified that the rare cases are indeed the more fatal cases,” Bayati said. “That is very important. We want to see what’s going on behind this pattern.”
AI Reveals Commonalities in Outliers
Bayati and Khattak used an unsupervised clustering algorithm to analyze the factors involved in more than 10,000 police-reported pedestrian crashes. Each report includes detailed information like the speed limit, lighting conditions, road surface conditions, and pedestrian position at the time of the crash.
They then directed the AI to divide the crashes into three categories relating to their distance from the core, or the cluster of ‘average’ scenarios. While only eight percent of core cases were fatal, nearly 37 percent of cases in the furthest edge category involved fatalities.
Analyzing the factors involved in those life-threatening ‘outlier’ crashes reveals the types of interventions that may be most effective at saving pedestrian lives.
“You can check the history of different locations and look for trends that lead to most crashes there,” Bayati explained. “We had a lot of crashes that happened on road shoulders, so we can consider installing sidewalks, speed bumps, or crosswalks in those locations to give pedestrians the opportunity to walk in safety.”
Strategic Interventions Save Lives
Part of what makes the edge cases so dangerous—and so rare—is that they occur when many risky factors coincide. That also means it is harder to design interventions against them.
For example, installing a sidewalk along an unlit stretch of a rural roadway will only make it safer during the day. Darkness was a common factor in the highest-risk crashes Bayati and Khattak analyzed, indicating that installing lighting with a sidewalk would lead to an even greater increase in safety.
These complex and rare scenarios can also be used to improve autonomous vehicle safety by exposing failure modes unlikely to appear in routine driving data. Autonomous cars need to be able to reliably sense pedestrians on the road shoulder even in darkness, for example.
“Nighttime crashes or freeway crashes require a lot of different kinds of countermeasures that are harder to implement,” said Khattak. “That might be another reason why safety improvements tend to focus on the most prevalent problems, like at urban intersections.”
Bayati hopes that this study will create a better understanding of how pedestrian risk is distributed across a transportation system and result in more impactful safety interventions.
“Pedestrian safety is a very important topic. It affects everyone,” she said. “I would be happy to have even a small role in creating a safer transportation system.”
Contact
Izzie Gall ([email protected])