OR YOU CAN JUST GET A GUN: Urban Alarm Backpack. “The Urban Crew bag from iSafe is a backpack with an alarm system built in. When you get into trouble, just trip the alarm by tugging on the grenade-like pin hidden in the left shoulder strap. The pair of 125-decibel sirens will fire off their piercing tones and a set of flashing strobes will stun and annoy anyone close to you.”
Or you could just buy a gun.