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?

Because the Nation sometimes responds to intentional killing, not human stupidity, or acts of God. Gets trickier as you go up the ladder.

I'm thinking that (at least in part) because the former is driven by malice and greed while the latter is not. There are other factors of course, but the aforementioned would figure prominently in the equation.
MSG (Join to see)

MSG.,
I found myself not viscerally effected, but saddened for what I suspect were folks not looking for a conflict.
This country and Europe brings in a problem. It's only a matter if time before they play their role again in growing government. It's a sick joke and the laugh will be on us for believing we can get along somehow.

It is the difference between accidents and intentional murder.

One is an accident, the other is an attack. Can't believe you had to ask.

One is intentional and pretty much out of our control. The other is not one incident but probably over 10,000 incidents.

It is an irrational emotional response to a national and very personal attack. Car accidents are not as personal to everyone at the same time and not as intentional. Even though we could probably have saved many more lives by investing in car safety, drivers education classes and repairing our national infrastructure.
Spending money on vets is not exciting to people. Spending money on weapons and getting the bad guys is extremely flashy and exciting. Very emotional and politicians like that especially when they can say they were part of it that ultimately got the bad guys. Of course bad guys can be a relative term considering on your history and perception. Most of the terrorist have little to no education and have been indoctrinated since birth to believe that everyone but them are evil. Most can only read a little except the leaders, who tell the masses what to believe. With little or no hope of ever having anything but what is given them by their leaders. What choice do they really have?
Seeing the desolate environment in Afghanistan. I can see why some might think we are evil for not sharing our tremendous wealth, when they have almost nothing and almost all Holy Scriptures teach to give to the poor. What is given rarely make it to the ones in greatest need but is taken by the leadership and they tell their followers how horrible Westerners are with our sinful perverse ways.
Then we don’t take the time to understand what drives their thoughts but just bomb them and blame them for what their leadership forces or tells them to do. Same thing happens throughout history. People suffer while the leadership and rich gain from it, with the least damage. Unless they get very unlucky.

Same thing happened in the American Colonies which is why we have The U.S.A. instead of a British colony today.

It's unplanned vs. planned deaths which contrasts the events.

I think it's because people don't want background checks, and restrictions on cars.

If a mans enemy has publicly threatened to wipe Israel of the map and wants to kill all the infidels. Traffic accidents that kill are not intentional, thus the name accident. Try to imagine every motorist in the states driving at 90 mph. That would be lunacy.