If, on the basis of an engineering and traffic investigation, the State Highway Administration 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 under its jurisdiction, it may establish a reasonable and safe maximum speed limit for that part of the highway. 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. Under this section, the State Highway Administration may: Establish a maximum speed limit to apply at all times or only at specified times; and Establish differing limits for different times of day, different types of vehicles, different weather conditions, or other factors bearing on safe speeds. An altered maximum speed limit established under this section is effective when posted on appropriate signs giving notice of the limit.