Bystanders must become emergency medical first responders in any Intentional Mass Casualty Event. A body can bleed out in as little as 3 minutes when arteries are severed. In the best of conditions, EMS may take 10 minutes to arrive.
In the Boston Marathon bombing, public citizens used whatever was available to apply pressure to wounds. They saved many lives.
If you are applying pressure understand it is a two-handed effort. Use whatever pressure is necessary to stop the bleeding. Use a tactical tourniquet when available. I carry a tactical trauma kit in my day bag. It goes with me whenever I go out.
See the latest recommendations for using tourniquets:
I bought my trama kit at Amazon for about $75