Seriously, don't be. Cambridge is a safe place. As the girl who lives next door to me was saying, in London, there are rapes and stuff all the time, it just doesn't get the air-play and so people don't blow it out of proportion. Risk assessment is key. If you were an attacker, would you jump someone in daylight at a busy time when everyone's going to lectures, so, in front of people, or would you do it when it was getting quiet and darker?