---
type: concept
tags: [parking, safety, devices]
sources: [parking-system-architecture]
updated: 2026-06-14
---

# Safety Principle: A Barrier Is Not a Door

A vehicle barrier must **not** be driven as a timed "door open for N ms" by the application — a
timed auto-close can **drop a boom on a vehicle or person**. (See [[parking-system-architecture]]
§5.)

- **Physical safety lives in the barrier operator's own firmware** — induction loops, anti-crush,
  auto-reverse. (Recommended barrier operators in the [[bom]] are chosen because they own this.)
- The application and any relay board **only ever express *intent* ("open")**; they never time or
  force a close against a vehicle. Reflected in the [[device-adapter-pattern]]'s `pulseOpen`.
- Holds **regardless of which relay device** is used — [[uhppote-controller]] or
  [[esp32-custom-controller]]. The ESP32 design restates it: "the ESP32 only signals intent"
  (see [[fail-state-safety]]).