Update SAFETY.md (#29129)

old-commit-hash: 1b90790b0f

docs/SAFETY.md

@@ -25,9 +25,12 @@ ensuring two main safety requirements.
    by stepping on the brake pedal or by pressing the cancel button.
 2. The vehicle must not alter its trajectory too quickly for the driver to safely
    react. This means that while the system is engaged, the actuators are constrained
-   to operate within reasonable limits.
+   to operate within reasonable limits[^1]. 
 
 For additional safety implementation details, refer to [panda safety model](https://github.com/commaai/panda#safety-model). For vehicle specific implementation of the safety concept, refer to [panda/board/safety/](https://github.com/commaai/panda/tree/master/board/safety).
 
 **Extra note**: comma.ai strongly discourages the use of openpilot forks with safety code either missing or
   not fully meeting the above requirements.
+
+[^1]: For these actuator limits we observe ISO11270 and ISO15622. Lateral limits described there translate to 0.9 seconds of maximum actuation to achieve a 1m lateral deviation. 
+