Out of curiosity, what areas would you say this to be true? Not denying there are dangerous areas, but I've only lived in SF for 2 years and haven't lived in many neighborhoods (1 year near the SoMissPo underpass and another near UCSF mission bay). The only one that comes off as dangerous at night to me would be the tenderloin or perhaps mid-market (although the latter is quickly becoming more and more expensive).