I’ve been studying transportation and traffic safety for much of the last three years. I’ve spent thousands of hours reading books on the subject, listening to expert interviews, and studying crash reports.

When investigating a fatal crash, there’s a very common story. “I didn’t see the pedestrian.” “That cyclist came out of nowhere.” “I didn’t see any people in the street.”

There are many reasons why traffic fatalities occur, but this story of “I didn’t see them” is a recurring theme. On average, seventy people will die today on America’s roads, killed by drivers who are sober and paying attention. The same number will die tomorrow, and the next day, and many of the drivers will say the same thing: “I didn’t see them.”

One could say that traffic violence occurs because of a national case of blindness. I’ve struggled to explain this “blindness factor”. I wanted to find a scientific explanation, and I finally found the root cause in a psychology book.

Daniel Kahneman won a Nobel prize for his work on psychology, and his book “Thinking Fast and Slow” is an expanded version of his Nobel-winning research. In his book he talks about “The Invisible Gorilla”, a study where people were shown a short film and instructed to count how often a basketball is passed back and forth. The experiment is designed to give viewers a “high cognitive load” – meaning that they are focused on the task and thinking very hard.

Daniel writes: “Halfway through the video, a woman wearing a gorilla suit appears, crosses the court, thumps her chest, and moves on. The gorilla is in view for 9 seconds. Many thousands of people have seen the video, and about half of them do not notice anything unusual.”

These people are fully paying attention, but half of them do not notice a gorilla on the basketball court for nine seconds. Daniel continues:

“… the most remarkable observation of [the] study is that people find its results very surprising. Indeed, the viewers who fail to see the gorilla are initially sure that it was not there – they cannot imagine missing such a striking event.”

People who are thinking hard are effectively blind, and even when presented with proof, they are unable to understand how they could miss the gorilla right in front of them.

This is the scientific explanation for the national blindness epidemic that is killing thousands of people a year: high cognitive load. When people are thinking hard about something, they will be effectively blind to things they would normally see. When people are thinking hard, they will be blind to a gorilla in plain sight.

When people are thinking hard, they will be blind to a child in front of them.

Cognitive load is not something we can eliminate. People will sometimes be busy reading street signs. Parents will sometimes have screaming kids in the back seat. Chatting with your passenger could be all it takes to give you a temporary case of blindness, and you wouldn’t realize it until it’s too late. There is no “more enforcement” or “better education” that will solve this mental blindness, and people who claim otherwise are ignoring science.

Are we doomed to suffering 40,000+ road deaths every year because of mental blindness? Thankfully, there are many improvements we can make to our streets to make them safer, even for those mentally blind drivers. In fact, there is one very simple way to make a huge difference: slow the motor vehicles.

The US Department of Transportation publishes a chart that compares vehicle speed to pedestrian fatalities. They found that at 20mph, the chances of pedestrian death is low: 10% or less. Pedestrians struck by a 30mph vehicle have a much higher chance of death: around 40%. At 40mph, the odds of killing a pedestrian with your vehicle goes above 80%.

The results are clear: 20mph and below is much safer for pedestrians and cyclists. Any vehicle over 20mph should be classified as high-speed motor traffic and considered to be deadly. Any place where pedestrians or cyclists are mixing with cars, the cars should have a speed limit of 20mph, and the road design should enforce that speed limit.

Separating high-speed traffic is not only more survivable, it will reduce cognitive load for drivers. At 20mph, a driver has much more time to see and react to the gorilla in front of them. Slower traffic also gives pedestrians more time to clear the street. At 20mph, a pedestrian crossing the street has more time to move out of the way of an oncoming vehicle.

There is an epidemic of blindness in the US transportation system that is killing thousands of people every year. Science shows us that humans are blind when they are thinking hard – blind enough to miss a gorilla right in front of them. Separating high-speed vehicles and their blind drivers from pedestrians and cyclists will reduce harm and save lives.

The longer we continue this mix of high-speed vehicles with pedestrians and cyclists, the longer we will continue killing “invisible” people on our streets.