I noticed an anomaly recently as I pondered a map of Interstate 5 covering California. Generally speaking, the custom in the United States is to drive on the right side of the road.(1) Just north of Santa Clarita near Castaic Lake, however, I-5 splits and switches that order. Vehicles driving steadily along on the right side suddenly flip to the left side of the divided highway and remain that way for several miles:
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It’s transparent to the driver in all practicality. Several hundred feet separate the northbound and southbound lanes. The average motorist probably doesn’t even notice the oddity as she flies by at high speed. Still, for a brief moment the California highway shares a common bond with a British motorway. The scenery doesn’t look much like the M1, but that’s as close as we’re going to get. Pretend we’re in the UK for a few moments.
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The effect can be observed rather clearly in this image recorded by Street View right after the split-and-switch. Notice the direction of the tractor-trailer on the hillside. Indeed, this southbound vehicle is on the left side of Interstate 5. That’s completely reversed from the norm.
I thought I’d discovered a remarkable sight. However, as I poked around I learned that it’s a situation both fairly well know and hardly unique. It’s even mentioned on Wikipedia’s Right and Left Hand Traffic page along with a several others:
Examples include the Golden State Freeway (I-5) in southern California during the descent/ascent of the Castaic Grade, several miles of Interstate 85 in Davidson County, North Carolina (map), a very brief section of Interstate 275 in St. Petersburg, Florida (map), the I-8 Freeway east of Yuma, AZ (map) state route 87 in Maricopa County, Arizona through Rincon Pass (map) and small parts of approaches to Interstate 64 running through Chesapeake, Virginia as part of the Hampton Roads Beltway.
Nuts. Foiled again.
I found a lot more information about the Interstate 5 anomaly than the Wikipedia paragraph so I feel somewhat redeemed. This section of roadway is called the Five Mile Grade and it climbs to one of the highest elevations along the entire length of I-5. The anomaly occurs because of the terrain. It’s a safety issue.
The current northbound lanes are the original part of an older highway: U.S. Route 99. It was a dangerous road with curves and uneven grades. Trucks heading downhill sometimes lost control. They caused horrific accidents including nasty head-on collisions.
Road engineers designed a second set of lanes on a separate path as they converted the highway to Interstate standards. They created a straighter road with a more consistent grade, a much safer proposition for vehicles traveling downhill. Logically they switched the "normal" flow of travel through the slot for the very best of reasons, to prevent accidents and save lives.
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A vestige of old U.S. Route 99 can still be seen along this stretch of Interstate 5. Notice that traffic is heading uphill towards the camera. Now look off to the right. That’s an old runaway truck ramp put in place when the old highway moved traffic in both directions. Back then of course vehicles drove to the right because it was the only highway through the slot. Sometimes a runaway truck needed a place to stop in an emergency. Thankfully it’s now superfluous.
You can learn more at the Historic US 99 Guide.