Inattention blindness "is a psychological lack of attention and is not associated with any vision defects or deficits. It may be further defined as the event in which an individual fails to recognize an unexpected stimulus that is in plain sight." This video called the Distracted Mind demonstrates inattention blindness perfectly.
Our minds are unable to truly multi-task. Instead, we can only switch from one task to another. Perhaps a better term for it is "task-switching". If you are talking on the phone, whether you are listening or speaking, your mind is engaged in cognitive tasks. Even if you aren't holding your phone, your mind is focused on this primary task of your conversation. Which means that if you are trying to talk about your dinner plans while you're driving, your mind is focused on the choice between pizza and pad thai, not what's in front of your windshield. A pedestrian in a crosswalk, a stop sign, or a brake light on the car in front of you each need just a split second of your attention to be noticed, but if you don't catch the visual cues in time, it may be too late to avoid a devastating accident.
As a personal injury lawyer who works with accident victims, I see a lot of people who are injured because of someone else's negligence. As I work through their case, I see medical records, talk to doctors, and walk with my clients through the mental anguish and trauma they're going through after the accident. It's not pretty, and these are people who are in pain long after the accident that takes just a few seconds. Sometimes, the unthinkable happens, and loved ones die. It's just not worth it.
Luckily, the law is on the side of people who are hurt because of someone else's mistake. You can get compensation and medical treatment that can help, but it can't undo what happened.