In Germany, 70% of the highways have to actual speed limit. This is currently idicated by no speed limit sign at all. However, this might also mean that the speed limit is unknown.
Instead, the "unlimited" speed sign could be used that is used in Germany to indicate that no speed limit applies anymore (link to an image: https://www.bussgeldkatalog.org/wp-content/uploads/282.png) This is sign 282.

This could be easily accomplished by either adding a checkbox to the editor to set that no speed limit applies, but this might requite some engineering.
An easier solution would be to only update the app to show the sign when the speed limit has a special value. The speed limit has to be saved internally in a database. The field might have a limit, e.g. 8bit. The maximum value possible would then be 255. So the app could be updated to show this sign when the speed limit is set to 255, which is also unrealistic to ever be used.

Alternatively, this behaviour could be hidden by a checkbox that internally sets the speed limit to 255 (or the maximum possible value)