▁ 1 9 5 6 ▁which ▁established ▁fund ing ▁for ▁the ▁inter state ▁highway ▁system ), ▁but ▁there ▁were ▁ 1 1 ▁specific ▁alternative ▁routes ▁available . ▁These ▁were ▁narrow ed ▁down ▁to ▁just ▁three , ▁with ▁the ▁chosen ▁route ▁following ▁the ▁route ▁of ▁Highway ▁ 9 1 ▁through ▁the ▁Missouri ▁C any on ▁and ▁past ▁Tower ▁Rock . ▁This ▁route ▁provided ▁both ▁lower ▁construction ▁and ▁maintenance ▁costs , ▁and ▁had ▁the ▁added ▁benefit ▁of ▁allowing ▁Highway ▁ 9 1 ▁to ▁act ▁as ▁a ▁front age ▁road . ▁State ▁pl ann ers ▁also ▁chose ▁to ▁build ▁a ▁four - lane ▁rather ▁than ▁two - lane ▁inter state , ▁arg uing ▁that ▁it ▁was ▁less ▁expensive ▁to ▁build ▁a ▁four - lane ▁road ▁now ▁than ▁to ▁w iden ▁the ▁route ▁in ▁the ▁future . ▁I - 1 5 ▁in ▁ob liter ated ▁parts ▁of ▁Highway ▁ 9 1 , ▁and ▁par alle led ▁it ▁in ▁other ▁places . ▁I - 1 5 ▁devi ated ▁from ▁the ▁route ▁of ▁Highway ▁ 9 1 ▁by ▁passing ▁Tower ▁Rock ▁to ▁the ▁east . ▁The ▁segment ▁of ▁I - 1 5 ▁between ▁Hel ena ▁and ▁Great ▁F alls ▁opened ▁in ▁ 1 9 6 8 . ▁ ▁About ▁the ▁park ▁ ▁Leg al ▁history ▁of ▁the ▁site ▁Tower ▁Rock ▁had ▁been ▁claimed ▁by ▁Native ▁Americans , ▁the ▁United ▁Kingdom , ▁Spain , ▁and ▁France ▁before ▁becoming ▁part ▁of ▁the ▁United ▁States ▁in ▁ 1 8 0 3 ▁through ▁the ▁Louisiana ▁P urchase . ▁The ▁territory ' s ▁name