Deterrent makes sense on further thought. The question just states unauthorized access. It doesn't state the intent of any unauthorized intruders. Deterrence is designed to reduce the occurrence of unintentional bystanders or unmotivated malicious agents from entering the site. Should the agent be motivated enough, a preventative measure is needed. But again, the question doesn't list intentions. Therefore this method works to limit the number of unauthorized visitors by weeding out everyone but the motivated, and the truly stupid.