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Appearance vs. Reality: “Dangerous” Intersections

Friday, June 14, 2019

As much as is possible, engineering is the study of what is. In a time when opinion can be elevated to truth and social media confirms all our fears, people are as prone as ever disregard facts for opinions. Good engineering demands we pay attention to the facts and then make a careful judgement on what to do. One of the clearest expressions of this is the study of “dangerous” intersection and how to improve them.

Over the past 12 years, HCE has completed crash analyses for over 150 intersections and roadway corridors across the six-county Chicago area. For each location, we reviewed police reports and crash data, created diagrams of crashes, and analyzed their probable causes. The goal was to improve safety and reduce congestion. For a large number of locations, the reason they were chosen became quickly apparent – severe injuries occurred or a significant number of a certain type of crash, such as those involving left turns, t-bones (more properly called “angle” collisions), or rear end crashes. But for some locations, no reason ever became apparent. Why was the location included? Why study a safe intersection? The answer is in the appearance.

We’ve all driven through an intersection and just knew it wasn’t safe. We can’t see around a corner. Traffic is coming too fast. That car waiting to turn blocks our view. Trucks are going too fast. Obviously this is death trap waiting to happen and someone needs to do something! Quick! Call IDOT! Call your state rep! Write to the local newspaper! Start a Facebook group! Or even start… an engineering blog! Our instincts to avoid danger and protect our loved ones are strong. But what happens when appearance isn’t in line with reality? When over the last five years that intersection, which is so obviously dangerous, has had only one crash – a bumper-to-bumper rear end with no injuries. Is the intersection really dangerous?

The reality of a situation needs to prevail and that takes a hard look at the facts. Maybe a traffic signal is needed, but maybe something as small as a warning sign is enough, or maybe the best answer is to do… nothing. Sometimes trying to “fix” a problem that isn’t there can make a safe intersection dangerous. A traffic signal will keep people from getting into those severe, dangerous t-bone crashes. BUT, it will also increase the occurrence of rear ends. Put in a red light camera and you might get even more rear ends. By addressing the appearance of danger, it is possible to create danger. Yes, rear end crashes result in fewer and less sever injuries, but if there’s never been an angle crash (t-bone), what exactly are we fixing? Why are we putting in a safety measure to prevent something that’s never happened only to cause new crashes?

Sometimes the truth of a situation is the opposite of our expectation and it takes experience and proper engineering to know the difference. And then, it takes even more experience and knowledge to explain it. Twelve years, 150 locations, and thousands of crashes later and I’m pretty sure we have the first part down. Now if only engineers were as good at communicating as we are with numbers…

Drive Safely!

Jeffrey T. Snape PE LEED AP