A denominator problem, not a compliance miracle

To compute what share of legally-required flood insurance is actually in force, FEMA needs two numbers per county: how many NFIP policies are active in the mapped flood zone, and how many residential structures exist there to be covered. The second number is the weak link — FEMA's own count of that denominator is missing for roughly 805 of the 3144 counties in this dataset, and for a smaller set (27 counties) the reported structure count is smaller than the number of active policies, which arithmetically produces a compliance rate over 100%.

What it looks like in real counties

Tarrant County, TX's raw NFIP figures would compute to roughly 228% compliance; Pinellas County, FL comes out over 1,000%. Neither means Tarrant or Pinellas homeowners are radically over-insured — it means FEMA's residential structure count for the mapped zone in those counties understates the real number, likely from a stale or incomplete parcel count feeding that denominator. This site reports those counties as "unreliable" rather than showing a rate that implies near-universal or impossible compliance.

Why not just hide the affected counties

Silently dropping roughly a quarter of US counties from a "flood insurance compliance" map would misrepresent coverage as complete when it isn't. Instead, every affected county still gets its hazard score, its voluntary (all-structure) insurance rate where available, and an honest note that the mandated-zone compliance figure specifically cannot be trusted — see any flagged county's page for the caveat in context.