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?

An attack is planned. Driving deaths are in large part due to choices. Do not text and drive, do not drink and drive, do not speed, do not drive at night when the bars close, keep your distance from cars weaving in traffic, etc. Sure you can be killed by a drunk driver, but there are choices that you make that can reduce your risk.