▁was ▁Den ver , ▁along ▁the ▁cor rid or ▁of ▁US   4 0 . ▁The ▁portion ▁across ▁the ▁Rock y ▁Mountains ▁was ▁added ▁to ▁the ▁plans , ▁after ▁lo bb ying ▁by ▁Colorado ▁officials , ▁following ▁the ▁US   6 ▁cor rid or . ▁The ▁orig ins ▁of ▁both ▁the ▁US   4 0 ▁and ▁US   6 ▁pre - date ▁the ▁U . S . ▁System ▁of ▁number ed ▁high ways , ▁using ▁established ▁trans cont inental ▁tra ils . ▁ ▁Ear lier ▁routes ▁Before ▁the ▁formation ▁of ▁the ▁United ▁States ▁Number ed ▁High ways , ▁the ▁U . S . ▁re lied ▁on ▁an ▁inform al ▁network ▁of ▁roads , ▁organized ▁by ▁various ▁compet ing ▁interests , ▁collect ively ▁called ▁the ▁auto ▁trail ▁system . ▁The ▁survey ors ▁of ▁most ▁tra ils ▁chose ▁either ▁South ▁Pass ▁in ▁Wy oming ▁or ▁a ▁southern ▁route ▁through ▁New ▁Mexico ▁to ▁traverse ▁the ▁Rock y ▁Mountains . ▁Both ▁options ▁were ▁less ▁form id able ▁than ▁the ▁higher ▁mountain ▁passes ▁in ▁Colorado , ▁but ▁left ▁the ▁state ▁without ▁a ▁trans cont inental ▁ar tery . ▁When ▁the ▁pl ann ers ▁of ▁the ▁Lincoln ▁Highway ▁also ▁decided ▁to ▁cross ▁the ▁Rock ies ▁in ▁Wy oming , ▁officials ▁pressed ▁for ▁a ▁loop ▁to ▁branch ▁from ▁the ▁main ▁route ▁in ▁Neb r aska , ▁enter ▁Colorado , ▁and ▁return ▁to ▁the ▁main ▁route ▁in ▁Wy oming . ▁While ▁the ▁Lincoln ▁Highway ▁was ▁briefly ▁rout ed ▁this ▁way , ▁the ▁loop ▁proved ▁imp ract ical ▁and ▁was ▁soon ▁removed