The right response to this constant, unending, low-level threat of sudden violence is to stay vigilant and reasonable, to clean up the damage, care for the injured, look out for one another, and elect leaders who will address the challenge with sanity and good judgment.
"Sane leaders" of "good judgement", like the ones who downplay cultural assimilation issues, promote nonsensical government agencies like the TSA, and find the motivation of bombers and stabbers to be opaque.