The most southerly point of Interstate 5 is at the United States-Mexico border at the San Ysidro border crossing, one of the busiest in the world. Beginning at the border in San Ysidro, which is part of the city of San Diego, as the John J. Montgomery Freeway, I-5 goes through the suburbs of National City and Chula Vista before reaching downtown San Diego. It then parallels the Pacific coastline, going through the northern suburbs of San Diego, bisecting the University of California, San Diego campus, before passing through the 28 miles (45 km) of Marine Corps Base Camp Pendleton in northern San Diego County. Here I-5 is also known as the San Diego Freeway.
At Dana Point, I-5 turns inland and heads due north through Mission Viejo to the El Toro Y interchange in southeastern Irvine. I-5 becomes the Santa Ana Freeway as it runs southeast to northwest, passing through major cities and suburbs in Orange and Southern Los Angeles counties. Angelenos refer to it as "the 5" or as the Santa Ana Freeway. From this point, the San Diego Freeway continues northward as I-405.
When the freeway reaches the East Los Angeles Interchange one mile (1.6 km) east of downtown Los Angeles, I-5 becomes the Golden State Freeway. The route continues through the San Fernando Valley and then crosses the Newhall Pass through the Santa Susana Mountains into the Santa Clarita Valley. For about a four-mile (6 km) stretch between Santa Clarita Valley and the Pyramid Lake, the northbound and southbound lanes separate and actually invert, with the southbound lanes being to the east of the northbound ones. Beginning at that point, the Golden State Freeway sharply rises to the north through the Grapevine to eventually reach the second-highest point of its entire length, the Tejon Pass (elevation 4,183 ft. (1,275 m)) through the Tehachapi Mountains. Path 26 power lines generally parallel the freeway along this stretch. The freeway then sharply descends for 12 miles (19 km) at Tejon Pass to around 1,600 feet (488 m) at Grapevine near the southernmost point of the San Joaquin Valley, approximately 30 miles (50 km) south of Bakersfield and 4 miles (6 km) south from where State Route 99 splits away from it in Wheeler Ridge.
I-5 south approaching Weed and Mt. Shasta.From Highway 99 to south of Tracy, I-5 skirts along the far more remote western edge of the great Central Valley, and thus here is removed from population centers such as Bakersfield and Fresno with other state highways providing connections. Interstate 580 splits off from I-5 at a point south of Tracy, providing a loop-route connection to the San Francisco Bay Area. After passing Tracy, I-5 heads due north through Stockton and Sacramento before turning due west to Woodland. At Woodland, the interstate heads northwest again towards Dunnigan, where it converges with Interstate 505.
From Dunnigan, I-5 skirts north along the western edge of the Sacramento Valley to Red Bluff. I-5 then enters the Shasta Cascade region, passing through Redding and Shasta Lake before climbing up to near the foot of Mount Shasta. The interstate then travels to Weed and Yreka before reaching the Oregon border.
About 3 miles (5 km) north of the California border, the highway crosses 4,310 foot (1,314 m) Siskiyou Summit, the highest point on I-5. Dropping down into Rogue River valley through Oregon's southern mountains and towns such as Ashland, Medford and Grants Pass. Turning north across three passes to the Umpqua Valley and through Roseburg, the mountains tend to turn into hills, and as it reaches Cottage Grove, the road enters the Willamette River Valley. At Eugene the highway intersects with the short Interstate 105. The interstate then heads almost due north, skirting Albany and Corvallis, and passes through Salem. There were plans to build a spur into Salem, Oregon, called Interstate 305.
In downtown Portland, I-5 runs adjacent to the Willamette River and passes by the Rose Garden Arena (center), and Oregon Convention Center (right).Just north of Salem, between mile marks 259 and 260 (closer to 260), placed in the median is a sign where the 45th parallel crosses I-5. It bears the words "45 Parallel half way between the Equator and the North Pole".
The highway then tracks a little to the northeast, Interstate 205 splits off south of the Portland metro area. From here it passes up through Tualatin and Tigard along former U.S. Route 99W before hitting the southern terminus of I-405 and the Marquam Bridge. Also planned was a spur in Portland off of I-405, called Interstate 505, but it was never built and has been removed from city plans. A stub of I-505 exists as a long exit ramp to U.S. Route 30.
After crossing the Willamette River on the Marquam Bridge, I-5 has junctions at the western terminus of Interstate 84 and the northern terminus of I-405. It then continues through the northern parts of the city of Portland, and crosses into Washington via the Interstate Bridge.
The highway begins in Washington over the Columbia River and drops down into the city of Vancouver. About seven miles (11 km) into the state, it reaches the northern terminus of I-205, which is on the eastern edge of the Portland-Vancouver metropolitan area. It then tracks north by northwest to Kelso and Longview, Washington, at which point it ceases paralleling a large bend of the Columbia. Continuing north through the Willapa Hills, the freeway eventually reaches Olympia, where it bends sharply east, after that it goes through Fort Lewis and by McChord AFB, then finally Tacoma where it bends sharply north again to reach Seattle. The Ship Canal Bridge carries it over Portage Bay in Seattle. The freeway makes its way out of the Seattle/Tacoma/Everett metro area, crosses the floodplains of three rivers, through the Skagit Valley and the Mount Vernon-Anacortes Metropolitan Area to the northern city of Bellingham to arrive at the Peace Arch Canadian border crossing between Blaine, Washington, and Surrey, British Columbia. Highway 99 continues northwesterly from the border into Vancouver, BC.