Most of the pre-1964 segments were constructed as U. highways rather than Interstates, and some were initially constructed as expressways that were later upgraded to freewas. The dates that predate 1964 are estimates and were determined based on older recordkeeping that was not as precise as the modern recordkeeping. and state routes along its route through the Mojave Desert, Inland Empire, and San Diego regions. The auxiliary lanes would enhance traffic flow between freeway entrances and exits.
However, some regional planners suggest that Interstate 15 may someday have a more even traffic flow in both directions during rush hours.The carpool lanes could potentially be operated as a toll facility similar to the manner in which California Transportation Ventures (CTV) operates the existing Interstate 15 express lanes.In September 1998, reporters from the San Diego Union-Tribune drove Interstate 15 for five days in a row between one mile south of Deer Springs Road (north of Escondido) south to California 52.Much of the slowing along Interstate 15 occurs near Miramar/Pomerado Road, Carroll Canyon Road, Carmel Mountain Road, and Camino Del Norte.Much of this traffic is due to new housing developments and the lack of major surface arterials that carry north-south traffic.