There are 70 primary interstate highways in the Dwight D. Eisenhower National System of Interstate and Defense Highways (commonly known as the Interstate Highway System, or IHS) is a network of freeways in the United States. The oldest parts of the interstate system date back to the 1950s, and the planning for the system began prior to World War II. To learn more about their history, read previous FreightWaves Classics articles here, here and here.

In this article and future articles, information about specific interstates will be explored. This series profiles the interstates in numeric order (Interstate 2  and Interstate 4 to date; there is no Interstate 1 and Interstate 3 is not yet active).

This map shows the route of I-5 from the Mexican to Canadian borders. (Image: (i5Highway.com)

As most people know, interstate highways are assigned one- or two-digit route numbers (such as I-10 or I-55). Associated “auxiliary” interstate highways receive three-digit route numbers (such as I-270, I-495, etc.). 

Generally, odd-numbered interstates run south-north, with lower numbers in the West and higher numbers in the East. Even-numbered interstates run west-east, with lower numbers in the South and higher numbers in the North. (This is the opposite of the national highway system, whose lowest numbered north-south routes are in the East, the highest numbered routes in the West. 

Interstate highways whose route numbers are divisible by 5 usually represent major coast-to-coast or border-to-border routes (for example, I-10 runs from Santa Monica, California, to Jacksonville, Florida, from the Pacific to Atlantic oceans). 

 Trucks parked at a rest stop along I-5. (Photo: Downtown Gal/Wikimedia Commons) 

Interstate 5 is unique

Interstate 5 (I-5) is the primary north–south interstate highway on the U.S. West Coast. It generally runs parallel to the Pacific coast in the states of California, Oregon and Washington. I-5’s total length is 1,381.29 miles.

Its southern origin is San Ysidro (San Diego), California, the nation’s busiest international border crossing; it ends at Blaine, Washington. I-5 connects all of the major population centers along or near the U.S. West Coast, including San Diego, Los Angeles, Sacramento, Portland and Seattle. I-5 also provides freeway connections to the San Francisco Bay area via I-580 and I-505.

What makes I-5 unique? It is the only continuous interstate highway that touches both the Mexican and the Canadian borders. When it reaches the Mexican border I-5 continues to Tijuana, Baja California, as Mexico Federal Highway 1. At the Canadian border, the highway continues to Vancouver as British Columbia Highway 99.

Trucks near the Otay Mesa Port of Entry in San Diego, CA. (Photo: Lance Cheung/USDA)

Interstate 5 in California

Of its nearly 1,400-mile length, more than half is in California (796.53 miles to be specific). It travels through (or very near) the following California cities – San Diego, Santa Ana, Los Angeles, Stockton, Sacramento, Red Bluff, Anderson, Redding and Yreka.

The southernmost point of I-5 is at the Mexican border at the San Ysidro Port of Entry, one of the busiest border crossings in the world. Beginning at the border in San Ysidro, which is part of the city of San Diego, I-5 goes through suburbs before downtown San Diego. Then it travels parallel to the Pacific coastline through San Diego’s northern suburbs. Where it merges with  I-805, it briefly becomes the widest Interstate in the United States. It then goes through the U.S. Marine Corps’ Camp Pendleton for 18 miles.

An I-5 sign for use in California.
(Image: Wikimedia Commons)

I-5 turns inland at Dana Point. It runs due north and then turns southeast to northwest as it passes through Orange County and southern Los Angeles County. Continuing through the San Fernando Valley, I-5 then crosses Newhall Pass in the Santa Susana Mountains and into the Santa Clarita Valley.

The terrain rises sharply to the north and reaches the second-highest point of its entire length at Tejon Pass (elevation 4,144 feet). It then travels through the Tehachapi Mountains. The highway then descends sharply to about 1,500 feet near the southernmost point of the San Joaquin Valley, which is about 30 miles south of Bakersfield.

From its intersection with SR 99, I-5 travels along the western edge of California’s Central Valley and then runs north through the cities of Stockton and Sacramento. Along its route it also connects with four interstate highways that take traffic to the San Francisco Bay area.

From Dunnigan, I-5 travels along the western edge of the Sacramento Valley to Red Bluff and then enters the Shasta Cascade region. While certainly not remote or desolate, this section of I-5 travels through among the least populated areas of the state.

A photo of I-5 showing the volume of vehicles on the interstate. (Photo: Caltrans)

Traffic in southern California

Along much of its length, Interstate 5 follows the route of US 99, one of the old national highways. When I-5 was originally built it was six lanes wide. It was then widened to a minimum of eight lanes throughout most of southern California. But with the region’s population explosion, it is not wide enough. San Diego County has more than 3.5 million residents; Orange County has more than 3.18 million residents; and Los Angeles County’s population is just under 10 million. Throughout southern California, an eight-lane Interstate 5 cannot handle the traffic of daily commuters, visitors and tourists, businesses, and truckers. 

In Orange County, a portion of Interstate 5 was rebuilt and substantially widened in the mid-2000s. While there are as many as 22 lanes, congestion can bring the highway to a standstill during rush hours.

In Los Angeles County, one of the key choke points on I-5 is its junction with I-10, US 101 and SR 60. A veritable maze of interchanges, ramps and flyovers, these roadways are largely unchanged since their original construction; they are overwhelmed by traffic virtually all day long.

Sections of what are now I-5 in southern California were built prior to Congress passing the Federal Highway Act of 1956, which provided the first funding for the Interstate Highway System. These sections included the Aliso Street Viaduct (built in 1948) and portions of former US 101. The sections of US 101 that became part of I-5 were decommissioned as part of US 101 in 1964. 

A photo of traffic on the I-5 in Mission Viejo, CA. (Photo: City of Mission Viejo)

U.S. Routes

In southern San Diego County, Interstate 5 replaced US 101. For most of the rest of its route northward, I-5 roughly parallels and replaced US 99, which was taken out of service in stages between 1964 and 1972. Parts of what was US 99 are now part of California State Route 99, Oregon Route 99, and Washington State Route 99. 

Even though I-5 runs very near the original route of US 99, large stretches of the freeway were aligned differently. In particular in the Central Valley, US 99 went through Bakersfield, Fresno and Modesto; I-5 does not. I-5 was constructed to the west of these cities to provide a faster, more direct north–south route.  This new route in the Central Valley was I-5’s final section to be built. It was dedicated and opened for traffic near Stockton on October 12, 1979. Canadian and Mexican representatives attended the dedication ceremony to commemorate the first contiguous freeway connecting the three North American countries.

The bypassed cities are served by SR 99. I-5 intersects only three U.S. routes in California (US 101, US 50 and US 97).

Traffic approaching an entrance to I-5 in Sacramento. (Photo: Caltrans)

FreightWaves is interested to know what you consider the best and worst stretches of interstate highway in the United States. Please send your choices to fwclassics@freightwaves.com.