This type of prison has relaxed perimeter security, sometimes without fences or other means of external security:

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Relaxed perimeter security is a defining feature of minimum security prisons. These institutions are typically for nonviolent offenders who are considered low risk and have short sentences. The purpose of minimum security facilities is to prepare inmates for reentry into society, which is why they often have work-release programs and other opportunities for community outreach.

This Type of Prison Has Relaxed Perimeter Security, Sometimes Without