Lived in Boston for the last four years, was there during the Marathon Bombing. I'm now traveling within Europe, and am pretty 100% confident it's safer somewhere like Poland or Croatia than any first tier American city.
But, really, who cares? Change of dying while commuting is catastrophic compared to a terrorist attack.
The Parisians had the right idea - day after the attacks, everyone fills up the cafes.
Which is the opposite of what we did in Boston - cower at home, while a couple kids with IEDs shut down an entire state.
Terrorists only win when the people they attack are chickenshit.
Each time the West gets hit, it gets a little harder to worry about the next one. Just like you might get into a car accident driving to work, some Islamist whackjob might blow himself up at a concert, market, or restaurant.
When the risk is going outside and doing things that involve other people, there's no different action to take as an individual. It becomes background risk.