When seconds count - your government bureaucracy is minutes (or longer) away. Remember, in large bureaucracies, and complicated systems with many rules and detailed laws (ie. government in the United States of 2012) the scope for individual, or small group action, grows smaller and smaller.
Cops only indirectly prevent crime in that by catching criminals AFTER the fact, they prevent crimes from ever happening. Some, anyway.
Potential victims are the ones that prevent crime. Be it banks with sophisiticated alarm systems and guards, or home owners with a Joe Biden signature model shotgun at the ready while they watch TV, or the woman getting off work late, walking through the dark parking lot, with her hand in her purse, clenching her .380.
Cops are mainly only active in trying to apprehend whoever did “it” to you. It’s YOUR job to prevent “it” from happening.