In reality, beefing up security at selected sites only results in shifting the threat to other more vulnerable targets where people congregate.  If it's not within an airport secured zone or a concert venue or school, then it will be at a popular supermarket, eatery, passenger bus, train, or retail store.  In a more-or-less free society you cannot install x-ray scanners or erect barbed wire barriers and electrified fences around everything.  Neither can most places afford round the clock security for the long haul, and even if one could afford it, it's the rare security setup relying on human beings that isn't eventually compromised by sheer boredom and fatigue.

So what does the average John and Jane Doe do about it?  Just expect to be involved in a mass emergency someday.  "This can't be happening to me!" is not a mindset conducive to survival.