ˌɛsˈɑːɹ wˈʌnhˈʌndɹɪd twˈɛnti wˈʌn wʌz ɐsˈaɪnd ɪn nˈaɪntiːnhˈʌndɹɪd sˈɛvənti sˈɪks ænd ɹᵻmˈuːvd fɹʌmðə pɹˈaɪmɚɹi stˈeɪt hˈaɪweɪ sˈɪstəm ɪn tˈuː θˈaʊzənd twˈɛnti wˈʌn, wɪððɪ ɛntˈaɪɚ lˈɛŋθ ʌvðə ɹˈaʊt bˌiːɪŋ ɹˌiːdˈɛzɪɡnˌeɪɾᵻd æz fɹˈʌntɪdʒ ɹˈoʊd ˌɛfˌɑːɹsˌiːˈeɪtʃ zˈiəɹoʊ ˈeɪt.
SR 121 was assigned in 1976 and removed from the primary state highway system in 2021, with the entire length of the route being redesignated as Frontage Road FRCH08.