Defendant on December 15, 1956 was arrested and charged with the crime of unlawfully driving his automobile on U. S. Highway; 77, a State Trunk Highway as defined by SDC Supp. 28.0209-1(57), in Moody County, South Dakota, at a speed in excess of a maximum speed limit of 60 miles an hour. He was found guilty by the Justice Court. He then appealed to the circuit court where he was again found guilty and sentenced to pay a fine of $50. He now appeals from this judgment and sentence.
*511Defendant admits that he was probably driving fo¿s car at a speed in excess of 60 miles an ;hour. He contends, however, that he is not guilty of the- crime as .charged as there was no., legally established speed zone where he was driving, limiting the speed to 60 miles an0-hour. ,
The .speed limit upon which the State relies was established by the State Highway Commission. In March of 1955 the State Highway Commission adopted the following resolution to become - effective on July 1,1955. “No vehicle travelling on South Dakota. State Trunk Highways shall exceed the speed of sixty (60) miles per hour-days; and fifty (50) miles per hour-nights; except in zones already restricted to a lesser limit.” This resolution was expressly adopted pursuant to the authority granted the State Highway Commission to establish speed zones on highways on the State Trunk Highway system under Chapter 187 of the Session Laws of 1941. (SDC Su-pp. 44.0303.).
Chapter 187 empowered the State Highway Commission “to determine and establish upon any State Trunk Highway or any part thereof limited speed zones, which speed limit shall constitute the maximum speed at which any person may drive or operate any vehicle upon such zoned street or highway or portion thereof so zoned and on which highway the maximum speed permissible in said zone has been conspicuously posted by signs adopted by the State Highway Commission.”
When the legislature empowered the State Highway Commission to establish the limited speed zones as provided in Section 2 of Chapter 187 of the Laws of 1941, the same statute expressly provided:
“It shall be unlawful at any time or at any place for any person to drive a vehicle upon the streets or highway at a speed in excess of 50 miles per hour during the hours when lights on vehicles are required by law or during the period when lights are not required at a speed in excess of 60 miles per hour.”
Clearly, there would have been no purpose for the legislature to have empowered the State Highway Comihission to *512establish the same speed limits. We cannot assume, however, that the legislature would enact a law without some purpose in view. Alexander v. Booth (Florida cases, 1951-1952), Fla., 56 So.2d 716. We believe the purpose of the legislature in so empowering the State Highway Commission to establish the speed zones as provided in Chapter 187 of the Laws of 1941 is clear. It expressly empowered the State Highway Commission to establish upon any State Trunk Highway or any part thereof limited speed zones because of the particular conditions existing therein. It did not delegate to the State Highway Commission the power to establish a statewide speed zone applicable to all State Trunk Highways.
The State insists, however, that Chapter 168 of the Laws of 1955 enlarged the powers of the State Highway Commission. This chapter amended Chapter 187 by eliminating the maximum speed limit of 50 miles per hour-nights and 60 miles per hour-days and inserted in its stead the following:
“It shall be unlawful at any time or at any place for any person to drive a vehicle upon the streets or highway at a speed greater than is reasonable and prudent under the conditions then existing, except as otherwise provided by law.”
The State argues that “By the elimination of this provision the authority and duty of establishing maximum speed limits devolved upon the State Highway Commission and other local authorities having jurisdiction over streets and highways.”
It will be noted that the resolution of the State Highway Commission was adopted prior to the effective date of Chapter 168 of the Laws of 1955 and adopted expressly pursuant to the powers conferred upon it by Chapter 187 of the Laws of 1941. Because of this it might be argued that it is not necessary for us to consider Chapter 168 as we could base our opinion entirely upon the powers of the State Highway Commission under the provisions of Chapter 187. The defendant, however, did not brief or argue this question.
*513We shall, therefore, consider this appeal as if the resolution were adopted pursuant to any authority granted under the provisions of Chapter 168 as well as Chapter 187.
Unless the powers of the Highway Commission were enlarged by the enactment of Chapter 168, such powers as it had would stem from Chapter 187 and would be in force in the same manner and for the same purposes before as well as after the enactment of Chapter 168. In accordance with the general rule of statutory construction “Where a section or part of a statute is amended, it is not to be considered as repealed and re-enacted in the amended form; but the portions which are not altered are to be considered as having been the law from the time when they were enacted and the new provisions are to be considered as having been enacted at the time of the amendment.” State ex rel. Nielson v. McCarty, 76 Idaho 153, 279 P.2d 879, 881. See also Krimmel v. Eielson, 406 Ill. 202, 92 N.E.2d 767; 82 C.J.S. Statutes § 380, p. 903.
Apparently the state is contending that by the enactment of Chapter 168 of the Laws of 1955 the legislature had no intent to eliminate the speed limits of 50 miles per hour- nights and 60 miles per hour-days but merely delegated its power to- fix such limits to the State Highway Commission. We are unable to so interpret Chapter 168. The amendment expressly eliminates the speed limit in miles per hour. If the legislature had intended.to delegate this power to the State Highway Commission, it appears to us it would have expressly done so.
The legislature by the enactment of Chapter 168 eliminated the speed limits of 50 miles per hour-nights and 60 miles per hour-days and substituted therefor the reasonable and prudent speed limit. This reasonable and prudent speed limit now existing upon the streets and highways of this state is not expressed in miles per hour, except where the statute or the State Highway Commission in the limited speed zones as construed herein, has so provided.
We are of the opinion that the State Highway Commission exceeded its authority in attempting to establish a maximum speed limit on all State Trunk Highways of *51460 miles per hour-days and 50 miles per hour-nights. The only speed limit on the highway on which defendant was driving was that which was reasonable or prudent under the conditions then existing. Therefore, defendant is not guilty of the crime of unlawfully driving his automobile in exc'ess of a maximum speed limit of 60 miles per hour as charged.
The defendant further argues that if the legislature intended to delegate to the State Highway Commission the power to make a zone out of the entire state, such would be unconstitutional delegation of legislative power. Under our holding it is not necessary that we consider this question. “A court is not warranted in passing on the constitutionality of a statute unless such a determination is essential to the disposition of the cause under consideration.” Friese v. Gulbrandson, 69 S.D. 179, 8 N.W.2d 438, 441.
Reversed.
HANSON, P. J., and SMITH and RENTTO, JJ., concur.