What's the best flash mob deterrent? Rapid police response, wrote Jerry Ratcliffe, chair of Temple's Criminal Justice Department and former London policeman, in an op-ed piece published in the Inquirer. "What could work is changing young criminals' perception that the benefits of mob violence outweigh the risks," he wrote. In addition to a rapid response, he explained, there must be visible, uniformed police patrols at likely flash-mob sites. A study published by Ratcliffe this week showed that foot patrols in crime hot spots reduced violence by 23 percent. "Some battles can only be won with boots on the ground," he wrote.