 The construction of Interstate 15 (I-15) in the Parowan area caused the state legislature to twice modify the alignment of SR-143. The first change came in 1968, due to I-15 being constructed to bypass Parowan west of town, rather than following the route of US-91 / SR-1 through the center of town. As a result, the state legislature moved SR-1 west onto the interstate alignment while keeping the old alignment in the state highway system. This was accomplished by designating the former southwestern part of SR-1 from I-15 at Summit to Center street in Parowan as a new highway (SR-38) and extending SR-143 through north Parowan up to I-15, incorporating the northwestern part of the former SR-1 alignment.
