Why do we respond so viscerally to a terror attack that kills say ... 20 but not to 30,000 killed by cars p/yr. Is it the attack itself?

Possibly because terrorism is a violent and war like intrusion into peoples lives whereas driving deaths are a preventable action that most people don't want to confront since we all drive and could potentially kill someone while doing so. Just my humble view.

MSG Davis, Sir; probably for the same reasons we neglect our Veterans, but cater to our felons..

Numbness.

The human capacity to get used to anything is scary. We are the most adaptive creature to ever live on planet Earth. So much so that we have adapted to ideas like Preventable Heart Disease, Car Accidents, and Cancer being commonplace... yet "statistically irrelevant" events are what make us angry, because we are so Arrogant that we believe we can control those...

That's a helluva good question and I'm sure there will be plenty of opinions in response. I suspect that the mental defenses that we've constructed to protect ourselves from the carnage on the highways simply don't apply to deaths inflicted by terrorists.

The National Safety Council tried for years to use the carnage on the highways to scare us into being better drivers. Sadly, it never worked. Every holiday they would predict that X number of people would perish in traffic accidents and they were amazingly accurate. Ultimately, they discovered (with the help of Dr. Harold Mendlesohn that when faced with the prediction, people simply dismissed the danger by assuming that it would be X number of "other people", not them. Ultimately, everyone was convinced that they were a better driver than everyone else on the road and their skill would save them. There is no reason to believe that attitude has changed.

Death by terrorism is different. We don't have special skills to protect us. We are simply random targets and that randomness is frightening.

As I said, this is just a guess based on some actual knowledge that I possess, but it is admittedly incomplete.

"The death of one man is a Tragedy, The death of Million's is a Statistic" Joseph Stalin.

Because the car accident that might kill you "seems" random...therefore, we "believe" in a combination of fate/control that will protect us. With a terrorist, the killing is intentional, targeted, and generally aimed at "all" of us...so less fate/control. The other "dark" interpretation? No one gets much attention for dying at 70 mph while texting...at least not like being cut down by a terrorist.

Do you suspect that an accident is not an incident? We have accidents and the car or cars involved didn't intentionally attack us. A purposeful attack to create terror is a measured incident. 30,000 didn't die all at once, but over a year and in different locations. A terrorist attack is not an accident. Huge difference. A plane that goes down because it was directed into a tower killing 1000s is significant. It happened spectacularly and the images are burned into the memory. Sadly, car accidents happen because people text, drive too fast, dui, and for the most part they aren't planned. I don't believe this is a valid comparison, the main difference being causation. We can prevent terror. We can decrease car accidents.

Because our brains are not designed to absorb that much trauma.

I would say it is the attack itself because it disrupts our sense of safety. Yes car accidents may be a leader in deaths, but we always hear that tidbit of information that convinces us we'll be safe. He was drinking, so I'll be safe as long as I don't drink. He was texting, so I'll be safe as long as I don't text. He wasn't wearing his seatbelt, so I'll be safe as long as I wear my seatbelt etc etc. Terror attacks on the other hand are not a part of daily life, at least in the US. We can write off a malfunction when an airplane goes down because things break eventually. We can't write off a hijacking because we don't know the other people we're on a flight with. We don't know if one of these people might have an ill intent, so it makes up paranoid. Our sense of safety is disrupted. When public events are disrupted by backpack bombs, it makes us hesitant to go out to public events. We might even distance ourselves from anyone carrying backpacks or any kind of baggage for fear of something unknown. It is not the death toll that makes terror attacks so profound. It's the sense of fear and the break in normalcy that really hits us.

One death is a tragedy. A thousand is a statistic.