What is now I-10 east of Los Angeles was generally part of the Atlantic and Pacific Highway, one of many transcontinental national auto trails. By 1926, when the United States Numbered Highways were assigned, the road across the desert east of Indio was unimproved, while the road from Indio west to San Bernardino (as well as various roads west to Los Angeles) was paved. In late 1926, US 99 was designated along the section of road from San Bernardino to Indio, where it turned south along present SR 86 on the west side of the Salton Sea. West of San Bernardino, US 99 ran to Los Angeles, concurrent with US 66 (via Pasadena) before turning north; this route to Los Angeles is north of the later alignment of I-10. The piece of this between San Bernardino and Indio was defined in 1915 as Legislative Route 26. (It continued south from Indio via El Centro to Heber. A 1931 extension took it south to Calexico on present SR 111.)