A fence that is 3 to 4 feet (approximately 1 meter) high serves primarily as a psychological and demarcation barrier. Its main purpose is to define a property boundary and discourage casual or opportunistic entry. This height is easily surmounted by a determined individual but is generally sufficient to deter someone who is not actively seeking to trespass. It effectively communicates that an area is private property without providing a significant physical obstacle, thus deterring only casual trespassers.

A. 8 feet: This height constitutes a high-security barrier designed to deter and delay determined intruders, not just casual ones.
C. 2 to 2.5 feet: This height is largely ornamental and can be stepped over easily, offering negligible deterrent value even to a casual trespasser.
D. 6 to 7 feet: This height presents a physical challenge that is difficult to climb, intended to prevent entry from more than just casual trespassers.

1. Fennelly, L. J. (2017). Effective Physical Security (5th ed.). Butterworth-Heinemann. In Chapter 10, "Physical Barriers," it is explained that fences under 4 feet are primarily for marking boundaries, while fences 6 feet or higher are considered true barriers intended to prevent climbing. This aligns with the 3-4 foot fence being a deterrent for casual trespassers.

2. Garcia, M. L. (2007). The Design and Evaluation of Physical Protection Systems (2nd ed.). Butterworth-Heinemann. Chapter 4, "The Physical Protection System," discusses the functions of deterrence, detection, and delay. A low fence (3-4 ft) primarily serves the deterrence function against an unmotivated adversary (i.e., a casual trespasser), as it offers minimal delay.

3. U.S. Army Corps of Engineers. (2019). Unified Facilities Criteria (UFC) 4-010-01: DoD Minimum Antiterrorism Standards for Buildings. Section B-2.2, "Fences and Walls," notes that for standoff distance credit, fences must be at least 7 ft (2.1 m) high, implying that shorter fences do not meet the standard for deterring determined adversaries, and thus serve a lesser, more demarcation-focused role.