1. Any person driving a motor vehicle on a highway shall drive the same at a careful and prudent speed not greater than nor less than is reasonable and proper, having due regard to the traffic, surface, and width of the highway and of any other conditions then existing, and no person shall drive any vehicle upon a highway at a speed greater than will permit the person to bring it to a stop within the assured clear distance ahead, such driver having the right to assume, however, that all persons using said highway will observe the law.
2. a. Unless otherwise provided by this section, or except as posted pursuant to sections 262.68 , 321.236, subsection 5 , section 321.288, subsection 2 , paragraph “f”, sections 321.289 , 321.290 , 321.293 , 321.295 , and 461A.36 , the following shall be the lawful speed and any speed in excess thereof shall be unlawful:
(1) Twenty miles per hour in any business district.
(2) Twenty-five miles per hour in any residence or school district.
(3) Forty-five miles per hour in any suburban district.
b. Each school district as defined in section 321.1, subsection 70 , shall be marked by distinctive signs as provided by the current manual of uniform traffic control devices adopted by the department and placed on the highway at the limits of such school district.
3. Unless otherwise provided in this section or by other speed restrictions, the speed limit for all vehicular traffic shall be fifty-five miles per hour.
4. A reasonable and proper speed is required, but not greater than fifty-five miles per hour at any time between sunrise and sunset, and not greater than fifty miles per hour at any time between sunset and sunrise, on secondary roads unless such roads are surfaced with concrete or asphalt or a combination of both, in which case the speed limits shall be the same as provided in subsection 3. When the board of supervisors of any county shall determine upon the basis of an engineering and traffic investigation that the speed limit on any secondary road is greater than is reasonable and proper under the conditions found to exist at any intersection or other place or upon any part of a secondary road, the board shall determine and declare a reasonable and proper speed limit at the intersection or other part of the secondary road. The speed limits as determined by the board of supervisors shall be effective when appropriate signs giving notice of the speed limits are erected by the board of supervisors at the intersection or other place or part of the highway.
5. a. Notwithstanding any other speed restrictions, the speed limit for all vehicular traffic on fully controlled-access, divided, multilaned highways is sixty-five miles per hour. However, the speed limit for all vehicular traffic on highways that are part of the interstate road system, as defined in section 306.3 , is seventy miles per hour. The department may establish a speed limit of sixty-five miles per hour on certain divided, multilaned highways not otherwise described in this paragraph.
b. The department, on its own motion or in response to a recommendation of a metropolitan or regional planning commission or council of governments, may establish a lower speed limit on a highway described in this subsection.
c. For the purposes of this subsection, “fully controlled-access highway” means a highway that gives preference to through traffic by providing access connections with selected public roads only and by prohibiting crossings at grade or direct private driveway connections.
d. A minimum speed may be established by the department on the highways referred to in this subsection if warranted by engineering and traffic investigations.
e. Any kind of vehicle, implement, or conveyance incapable of attaining and maintaining a speed of forty miles per hour shall be prohibited from using the interstate road system.
6. Notwithstanding any other speed restrictions, a self-propelled implement of husbandry equipped with flotation tires that is designed to be loaded and operated in the field and used exclusively for the application of organic or inorganic plant food materials, agricultural limestone, or agricultural chemicals shall not be operated on a highway at a speed in excess of thirty-five miles per hour.
7. Notwithstanding any other speed restrictions, the speed limit for all vehicular traffic on a street or highway on which a cattle guard is installed pursuant to section 314.30 is fifteen miles per hour between the point at which the cattle guard is installed and the point at which the street or highway terminates in a dead end.
8. A person who violates this section for excessive speed in violation of a speed limit commits a simple misdemeanor punishable as a scheduled violation under section 805.8A, subsection 5 . A person who operates a school bus at a speed which exceeds a limit established under this section by ten miles per hour or less commits a simple misdemeanor punishable as a scheduled violation under section 805.8A, subsection 10 . A person who violates any other provision of this section commits a simple misdemeanor.
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