House numbers should be clear enough so that police, the fire department, paramedics, etc., can quickly locate properties in an emergency. House numbers are often the only way that first-responders can identify their intended destinations. A number of jurisdictions have begun enforcing laws through strict fines for homeowners who do not comply with laws that impose requirements for house numbers.
In order for house numbers to be visible from the street, InterNACHI advises that they should:
Inspect the address or street number to determine that it is visible from the street with numbers in contrast to their background.
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