Boston Area

Thoughts go out to them, and if you're in the area, keep your head on a swivel and stay safe.  Sounds like one down (good... driving(?) by the Police, shot by the police, then possibly run over by his brother as he made his escape.  Sounds like he could have been wearing a Suicide Vest IED type device.  No doubt hoped to take quite a few with him.)

Tools

In the spirit of the Boy Scouts, Be Prepared. Have the tools you need, and the training required to use them.  Carry these tools when you think you may need them, no matter how small the chance seems.

The C-A-T Combat Application Tourniquet is, in my opinion, one of the better options out there. It's not perfect (the windlass was known to break on older versions) but it's easy to use with the correct training (part of the reason there are two of them on my person as I type.)

While this may not be something everyone should carry, it is something worth looking into for first responders, especially LEOs or other personnel working security at large events.  I know I will continue to carry one at least while working when I get home.