Which measure is least effective against physical intrusions?

Signage is considered the least effective measure against physical intrusions because it primarily serves as a deterrent rather than a barrier or active response mechanism. While displaying warnings or telling people that they are on private property can discourage some intruders, it does not provide any physical protection or immediate response capability. Therefore, it may not prevent an intrusion from occurring in the first place.

In contrast, fencing creates a physical barrier that can prevent unauthorized access, making it a more effective deterrent. Sensors can detect movement and provide alerts, while alarms can notify security personnel or law enforcement in real-time, both of which actively respond to intrusions and help in preventing unauthorized access. These measures offer more robust protection against physical threats when compared to the passive nature of signage.