The efficacy is measured for a public health intervention related to community-based planning for population protection measures (PPMs; ie, shelter-in-place and evacuation). This is a mixed (qualitative and quantitative) prospective study of intervention efficacy, measured in terms of usability related to effectiveness, efficiency, satisfaction, and degree of community engagement. Two municipalities in the Commonwealth of Puerto Rico are included. Community members consisting of individuals; traditional leaders; federal, territorial, and municipal emergency managers; municipal mayors; National Guard; territorial departments of education, health, housing, public works, and transportation; health care; police; Emergency Medical Services; faith-based organizations; nongovernmental organizations (NGOs); and the private sector. The intervention included four community convenings: one for risk communication; two for plan-writing; and one tabletop exercise (TTX). This study analyzed data collected from the project work plan; participant rosters; participant surveys; workshop outputs; and focus group interviews. Efficacy was measured in terms of ISO 9241-11, an international standard for usability that includes effectiveness, efficiency, user satisfaction, and "freedom from risk" among users. Degree of engagement was considered an indicator of "freedom from risk," measurable through workshop attendance. Two separate communities drafted and exercised ~60-page-long population protection plans, each within 14.5 hours. Plan-writing workshops completed 100% of plan objectives and activities. Efficiency rates were nearly the same in both communities. Interviews and surveys indicated high degrees of community satisfaction. Engagement was consistent among community members and variable among governmental officials. Frontline communities have successfully demonstrated the ability to understand the environmental health hazards in their own community; rapidly write consensus-based plans for PPMs; participate in an objective-based TTX; and perform these activities in a bi-lingual setting. This intervention appears to be efficacious for public use in the rapid development of community-based PPMs.