Except as provided in paragraphs (3) through (6) of this subsection, if, on the basis of an engineering and traffic investigation, a local authority determines that any maximum speed limit specified in this subtitle is greater or less than reasonable or safe under existing conditions on any part of a highway in its jurisdiction, it may establish a reasonable and safe maximum speed limit for that part of the highway, which may: Decrease the limit at an intersection; Increase the limit in an urban district to not more than 50 miles per hour; Decrease the limit in an urban district; or Decrease the limit outside an urban district to not less than 25 miles per hour. An engineering and traffic investigation is not required to conform a posted maximum speed limit in effect on December 31, 1974, to a different limit specified in § 21–801.1(b) of this subtitle. Calvert County may decrease the maximum speed limit to not less than 15 miles per hour on Lore Road and, except for Solomons Island Road, each highway south of Lore Road without performing an engineering and traffic investigation, regardless of whether the highway is inside an urban district. This paragraph applies only to: Anne Arundel County and Montgomery County; and Municipalities located in Anne Arundel County and Montgomery County. A local authority may decrease the maximum speed limit to not less than 15 miles per hour on a highway only after performing an engineering and traffic investigation. A local authority may not implement a new speed monitoring system to enforce speed limits on any portion of a highway for which the speed limit has been decreased under this paragraph. Baltimore City may, without performing an engineering and traffic investigation: Decrease the maximum speed limit on a highway under its jurisdiction; or Increase to a previously established level the maximum speed limit on a highway under its jurisdiction. A municipality located in Queen Anne’s County may decrease the maximum speed limit to not less than 15 miles per hour on a highway without performing an engineering and traffic investigation. In school zones designated and posted by the local authorities of any county: The county may decrease the maximum speed limit to 15 miles per hour during school hours, provided the county pays the cost of placing and maintaining the necessary signs; and Any municipality within each county may decrease the maximum speed limit in a school zone within the municipality to 15 miles per hour during school hours, provided the municipality pays the cost of placing and maintaining the necessary signs. An altered maximum speed limit established under this section is effective when posted on appropriate signs giving notice of the limit. Except in Baltimore City, any alteration by a local authority of a maximum speed limit on a part or extension of a State highway is not effective until it is approved by the State Highway Administration. If a local authority determines that any maximum speed limit specified in this subtitle is greater than reasonable or safe in an alley in its jurisdiction, the local authority may establish a reasonable and safe maximum speed limit for the alley. The local authority shall post a speed limit established under this subsection on appropriate signs giving notice of the speed limit.