being able to yell has never prevented attacks to any large degree. Effective evasion or effective defense are the only reasonable approaches. If you are within reach of an assailant then evasion has been eliminated so defense is the appropriate response. Car alarms don't cause bystanders to call the police and neither do personal alarms. The assailant can take your noise-maker away and finish the intended task long before the arrival of help. Bystanders will not suddenly become heroes just because you make a mechanical noise instead of a scream. In america atleast the sad truth is that bystanders routinely, well, stand-by instead of taking the risks implied by intervening in anyway. If you wish to deter an attack, be prepared, trained and equipped to inflict pain or injury to your assailant. Short of that is polyannish and niave. The false sense of security provided by these personal alarms might be more dangerous in that while carrying one someone might think that they are safe walking out to their car alone at 4:00 am after the bar has closed rather than going in a group or some similar foolishness.

Also, The police and the Student Union are both rabid gun-control activist lobbiest for the most part and would rather you be disarmed, at risk and eventually dead, than risk the potential that you shoot someone undeserving and they be blamed for advocating that you be armed.