It was Montana. See picture from 1999 below. Unfortunately some numbskull brought the case to court and it was determined that the standards were subjective, so they were eliminated.
Freeper BlackElk (R.I.P.) told me that the Merritt Parkway in Connecticut (nut exactly rural) had a similar speed limit way back ('50s?), until someone was clocked at 105, and went to court, claiming he was a professionally skilled driver, and that for him, 105 WAS reasonable and prudent. He beat the charge, CT changed the limit.
