Interstate 8 (I-8) is an interstate highway that stretches from the southern edge of Mission Bay in San Diego, California, to its junction with I-10, just southeast of Casa Grande, Arizona. The route is 348.25 miles long.

Interesting facts

In California, I-8 runs through the San Diego metropolitan area as the Ocean Beach Freeway and the Mission Valley Freeway before traversing the Cuyamaca Mountains and the Imperial Valley. It crosses the Colorado River into Arizona, and goes through Yuma, crosses the Sonoran Desert and ends at its intersection with I-10 near Casa Grande, in between Phoenix and Tucson.

While the highway is less than 350 miles long, it passes through urban, suburban, rural areas, as well as two Native American reservations. It also traverses many different terrains; it begins very close to the Pacific Ocean, crosses the Colorado River and runs through two national forests, the Imperial Sand Dunes Recreation Area and desert terrain. I-8 runs parallel to the All-American Canal for about 55 miles across the desert, including the Sonoran Desert National Monument. In some areas of eastern Imperial County the Mexican border is less than half a mile south of the Interstate.

Interstate 8 in the Arizona desert. (Photo: Arizona Department of Transportation)

I-8 also has the distinction of having the lowest elevation of any of the interstates; it dips to 52 feet below sea level in California’s Imperial Valley near El Centro. Interstate 8 also travels through several mountain passes and its highest elevation is 4,000 feet at Carpenter Summit. 

The interstate straddles the line between San Diego and Imperial counties for a few miles before turning east. When it reaches the Mountain Springs/In Ko Pah grade, I-8 is routed through two canyons – Devils Canyon for westbound traffic and In-Ko-Pah Gorge for eastbound traffic – and it descends 3,000 feet in 11 miles. In places, the median is over 1.5 miles wide. This section of I-8 experiences high winds in the canyons, which has led to closure of the road at times. In 1966, the California Highway Patrol estimated that winds reached 100 miles per hour in the area.

A bridge over Devils Canyon on I-8. (Photo:

In Arizona, as the interstate takes a course west of Wellton, it turns northeast, paralleling the Gila River. I-8 then passes along the northern edge of the Barry M. Goldwater Air Force Range and south of the Yuma Proving Ground. 

Construction highlights

In May 1963, completion of Interstate 8 was the second highest priority of all road construction in California, according to the Highway Development Association. The number one priority was  Interstate 5.

The origins of I-8 in San Diego go back to 1957 when the U.S. Route 80 (US 80) freeway construction began. US 80 was built east from US 101 to El Cajon by the early 1960s. US 80 through San Diego was largely complete when it was renumbered as I-8 in the 1964 state highway renumbering. When first planned, I-8 was going to end at its intersection with I-5. It did until 1971, when the Ocean Beach Freeway was renumbered from SR 109 to I-8, which extended I-8 west of I-5.

An aerial photo of I-8 a few decades ago. (Photo:

East of San Diego, what had been US 80 was slowly replaced by I-8 as construction took place in the Imperial Valley. In Arizona, construction began in the early 1960s. By 1975 the California portion of I-8 was completed, and the Arizona portion of the interstate was finished in 1977. However, construction of the bridge over the Colorado River did not finish until 1978. 

Since then, as is true with most of the nation’s interstates, widening and improvement construction has occurred ever since. In addition, a portion of the interstate in Imperial County was rebuilt after it was damaged by Hurricane Kathleen.

Like many of the interstate highways (particularly as they run through major cities and other urban areas), there is often more traffic than the road can hold. I-8 is no exception. Back in February 1981, the California Department of Transportation (Caltrans) announced that I-8 east of I-805 was the busiest highway in the region. Then in 1987, Caltrans determined that I-8 West between College Avenue and Waring Road had the highest ratio of cars to number of lanes in the world, at 2,400 cars per hour.

A recent photo of traffic along a section of i-8 in San Diego. (Photo: Caltrans)

According to the San Diego Union newspaper in May 1965, the construction of I-8 through the canyons mentioned above was “through some of the most rugged, hottest sections of San Diego and Imperial counties,” in addition to concern about potential landslides. The highway’s westbound lanes were built first, and were temporarily used by both directions of traffic while the old highway was rebuilt as the eastbound lanes. The Los Angeles Times described the section of roadway east of Mountain Springs in this way: “Through it the freeway engineers have hacked two separate roadways not even in sight of each other, but so overpowering in the sheer magnitude of the cuts through the mountains that it is almost impossible to believe human beings could have so overpowered hostile nature …” Site access for the construction crews was very difficult, and many slopes had to be stabilized. In the summer, temperatures reached 120°F and 4 °F (−16 °C) in winter; wind gusts blew up to 80 miles per hour.  

On one of the sections of roadway in a more urban area of San Diego, about 5.5 million cubic yards of dirt and rock were generated by three construction projects; half of a mountain was  removed with a million pounds of dynamite. 

A truck approaches an entrance to I-8 West. (Photo:

The first roads

Like many highways and interstates in the U.S., when I-8 was built it either was built on parts of earlier roads or very near them.

In 1912 the first road over the Cuyamaca Mountains was dedicated. It was narrow and wound through the mountains. This trip could take up to four hours, and depending on the time of year could frequently result in the radiator boiling over, flat tires, or broken fan belts. Or inclement weather could end with cars stuck in mud. 

In 1926 the road was paved and opened in 1927. Road remnants were still existent in the late 20th century. In the early 1930s another road was built to remove curves and widen lanes. This two-lane road still had numerous switchbacks; one was known as “Dead Man’s Curve.” 

When I-8 was built it was placed atop some of the 1930s roadbed. According to the San Diego Union, the lack of a good road to San Diego caused increased development in Los Angeles, which caused it to become the trade and population center of southern California instead of San Diego.

Decades later, I-8 changed the drive between San Diego and El Centro from a 3.5-hour ordeal to less than two hours.  

A plank road made of pieces of wood that were tied together was the first road across the Imperial Valley to Yuma. These were later replaced by US 80 across California and part of Arizona. 

Between Yuma and Gila Bend in Arizona, I-8 runs along what was once the Gila Trail and the Butterfield Overland Mail Company line. This was a stagecoach line that connected Tipton, Missouri, and San Francisco. It transported passengers across the country at a stately five miles per hour between 1858 and 1861. Later, the Southern Pacific Railroad was built parallel to the Gila River east of Fortuna. The railroad was completed to Yuma by 1877 and to Tucson by 1880. 

In Arizona, US 80 was part of the system of highways proposed by the states in 1921. In 1926 that came to pass; however the road was paved with gravel and not asphalt. What would become I-8 some 40 years later was paved from Yuma to Casa Grande by 1935. Nonetheless, drivers were cautioned to carry spare fan belts, radiator hoses and additional drinking water for the journey crossing the desert.

The Ocean-to-Ocean Bridge was the first bridge for autos/trucks across the Colorado River near the route of the current I-8. (Photo: Wikipedia)

In 1915 the Ocean-to-Ocean Bridge across the Colorado River was opened in 1915. It was replaced in 1956 and was used until the I-8 bridge was built. The new bridge was dedicated on August 18, 1978 and opened to traffic on September 20; this completed I-8 from San Diego to Casa Grande. 

By the time the California sections of I-8 were completed, the average cost was $1 million per mile (about $4.1 million today).