Don't know what kind of road you were on at the time, but this may provide some wiggle room:
§17C-6-3. When local authorities may alter speed limits.
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(c) Authority to decrease fifty-five mile limit. -- Whenever local authorities within their respective jurisdictions determine upon the basis of an engineering and traffic investigation that the speed under this chapter upon open country highway outside a business or residence district is greater than is reasonable or safe under the conditions found to exist upon such street or highway, the local authority may determine and declare a reasonable and safe limit thereon but in no event less than thirty-five miles per hour and subject to subsection
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(e) Alteration of limits on state highways in municipalities. -- Alteration of limits on state highways or extensions thereof in a municipality by local authorities shall not be effective until such alteration has been approved by the commissioner of highways.