The DOT looks at several criteria in setting speed limits.  These include roadway alignment, sight distance, 85th percentile speed (a weighted average speed), crash history, and development.  For any sign to be effective, it must command the respect of road users.  This means speed limits must be reasonable and enforced.  The NCDOT is responsible for establishing speed limits, but local police, the State Highway Patrol, and Sheriff 's Departments have enforcement responsibility.