It's about as long as Hadrian's Wall
(a.k.a. Roman Wall, a.k.a. The Wall, a.k.a. Vallo di Adriano, a.k.a. Vallum Aelium, a.k.a. Severus' Wall) (from North Tyneside, Tyne and Wear, England, United Kingdom to Cumbria, North West England, United Kingdom)
Hadrian's Wall, which crossed England form the North Sea to the Irish Sea during the time of Roman rule, measures 120 km (about 79 Roman miles). Roman soldiers — at one time up to 10,000 of them — only occupied Great Britain for about 246 years after the wall was built, after which time the local Brits began to take residence the wall's garrisons.
It's about one-and-one-tenth times as long as The 405
In other words, 123 kilometers is 1.0554 times the length of The 405, and the length of The 405 is 0.947510 times that amount.(a.k.a. I-405, Interstate 405, San DIego Freeway) (Southern California)
Running from Irvine to San Fernando, The 405 runs a total of 116.541 km. The 405 is the most heavily-trafficed freeway in the United States, with an average speed as low as 8 kph during rush hours.
It's about one-and-one-fifth times as long as The English Channel
In other words, 123 kilometers is 1.150 times the length of The English Channel, and the length of The English Channel is 0.870 times that amount.(average width between Isle of Ushant and The Walde Lighthouse) (La Manche, Ärmelkanal, Mor Breizh, Mor Bretannek) (a.k.a. The Channel)
The English Channel measures between 180 km to 34 km, narrowing as it flows northeast. The world speed record for a Channel crossing by swimmer was set by Australian Trent Grimsey in 2012, with a time of 6 hours, 55 minutes.
It's about one-and-two-fifths times as long as The Panama Canal
In other words, the length of The Panama Canal is 0.710 times 123 kilometers.(Canal de Panamá) (Panama)
Connecting the Atlantic and Pacific Oceans across the Isthmus of Panama, the Canal measures 88 kilometers in length. Ships passing through the Canal are raised (and then lowered) 0.026 kilometers above sea level through the lock-and-dam system.
It's about three-tenths as long as The Grand Canyon
In other words, 123 kilometers is 0.2760 times the length of The Grand Canyon, and the length of The Grand Canyon is 3.620 times that amount.(a.k.a. Gran Cañón) (Arizona) (Colorado River miles measure)
The Grand Canyon has a total length of 446 km along the course of the Colorado River at its bottom. Having opened in early 2007, the Grand Canyon Skywalk allows tourists to stand on a horseshoe-shaped glass walkway 1.10 kilometers above the floor of Granite Gorge.
It's about four-and-a-half times as tall as Olympus Mons
In other words, the height of Olympus Mons is 0.220 times 123 kilometers.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 27 km. The mountain has been known to astronomers since the nineteenth century because it is tall enough to rise above Mars' frequent dust storms.
It's about one-tenth as long as The Rhine
In other words, 123 kilometers is 0.0932 times the length of The Rhine, and the length of The Rhine is 10.70 times that amount.(a.k.a. Rhein, a.k.a. Rijn, a.k.a. Rhin, a.k.a. Rain, a.k.a. Reno, a.k.a. Rhenus, a.k.a. Ryn) (from near Reichenau, Switzerland through Hoek van Holland, Rotterdam, Netherlands)
The Rhine river runs 1,320 km in total length. The World War II Battle of Arnhem, as memorialized in the 1974 book and 1977 movie A Bridge Too Far, was the first of several attempts to capture key bridges across the Rhine, beginning with the Arnhem Bridge in the Netherlands.