Verified by 
Could a cop run past an assault and fail to notice it? Maybe.
Kenny Conley, a Boston police officer, was chasing a shooting suspect. During the chase, he ran past a brutal assault. Other cops were attacking an undercover cop because they mistakenly believed he was involved in the crime. Conley was eventually asked to testify about what he saw of the assault. He claimed he saw nothing. How could he have missed the fight? He ran right past it. Even though it was night, there were plenty of streetlights. The fight would have been easily visible. Jurors didn't believe Conley's claim that he didn't see the fight. Instead they assumed he was part of a cover up of police misconduct. They convicted Conley of perjury and obstruction of justice.
Chabris, Weinberg, Fontaine, and Simons (2011) wondered if this could have been an instance of inattentional blindness. Inattentional blindness is failing to notice something that you would expect to notice because your attention is focused on another task. Being creative researchers, Chabris and colleagues recreated the situation as part of a real-world experiment. They asked people to run around a ¼ mile route on campus while chasing one of the researchers. The experimenters asked the participants to stay about 30 feet behind the researcher and to count the number of times he patted his head. Part-way through the route, they ran right past a staged fight about 25 feet off the route. The experiment was conducted at night, but there were plenty of streetlights (psychologists insist on conducting their research in safe environments). The fight would have been visible for about 15 seconds and the fighters were making a bit of noise.
After running the route, the participants were asked if they had seen the fight. Only 35% noticed the fight. Chabris et al. then replicated the experiment during daytime. They put the participants in either a low attention load condition (simply chase the guy) or a more difficult attention load condition (count the number of times he touches his head with both his left and his right hand). In daylight with the low attention load, 72% noticed the fight. But with high attention load, only 42% noticed the fight. Even in broad daylight, people can fail to notice a fight that occurs right beside their path if their attention is occupied.
In this paper and their other work, Simons and Chabris have argued that people have an illusion of awareness. We believe that we are aware of the things that happen around us. Instead, we miss a lot. Things happen around us, and sometimes right in front of us, and we fail to become aware of the event. We believe we are aware and are doing fine because we are unaware of all the things we are missing, like fights and unicycling clowns and traffic signals. We are surprised when people point out the things we miss.
This situation shares features with what social psychologists call the bystander effect. In the bystander effect, people notice someone in trouble but don't help. They fail to stop and help because there are so many other people around that they assume someone else will help. For example, how many times have you driven past someone stalled out on a busy road? Someone will stop to help, right? Maybe in some of these bystander cases, people actually fail to even notice the situation. They aren't failing to help; they are instead failing to see.
Although Conley was originally convicted of perjury and obstruction, the verdict was overturned on appeal. Chabris et al. noted, however, that the role of inattentional blindness was not mentioned in either the original trial or the appeal.
