It started out as it often does through a chance encounter with a roadmap anomaly. I happened to be examining a stretch of highway online. Then I spied an uncharacteristically wide split between the westbound and eastbound lanes of Interstate 84 directly outside of Pendleton, Oregon.
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It seemed quite remarkable. A mountain ridge forced opposing lanes to split and separate by an unusual distance. I used mapping tools to measure a line directly across the opening to estimate the greatest separation, which I calculated at around 1.7 miles (2.7 kilometers). This required a level of subjectivity. I attempted to measure between equivalent points on opposing lanes, measured almost horizontally in this instance. Obviously longer distances could be calculated by angling the line. That felt like cheating though. I decided to stick with my original estimate, giving or taking a small amount to account for eyeball-level accuracy, and wondered whether I could discover roads with greater separations.
Likely candidates went by different names such as
duel carriageway, freeway, motorway or divided highway, based upon linguistic variations of English-speaking nations. Likewise the separation between opposing lanes might be called a central reservation in the United Kingdom or a median strip in the United States, as examples. These terms could be mixed-and-matched to target online searches for better candidates.
Let’s dispel with the knee-jerk candidate right away.
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An Intertubes mythology has coalesced around Brasília’s Monumental Axis ("Eixo Monumental"). This impressive roadway defined a core for Brazil’s capital city, a feat certainly worthy of recognition. This also created an uncorroborated notion that Monumental Axis was either the widest road or the widest central reservation on the planet. Clearly it is neither. I measured the median at 0.25 miles (0.4 km). That’s a wide spot for Brazil perhaps although completely unremarkable when compared to the Pendleton split.
I wasn’t the first to ponder this question. Others blazed a trail before me and recorded numerous instances, including the Pendleton example I felt so smug about "discovering" a few days ago. A roadfan discussion and an FAQ proved to be particularly helpful so I stole from those sources liberally. Examples going forward should be ascribed to people who mentioned them there. I was too lazy to find any other instances on my own.
My crude measurements suggested co-champions, one in Canada and one in México.
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The Trans-Canada Highway presented a paradox, a 2.6 mile (4.2 km) separation for no apparent geographic reason, located outside of Ernfold, Saskatchewan. There weren’t any mountains to bypass. Muskeg wouldn’t be an issue this far south. I did notice a number of small lakes and ponds within the vicinity although those should have been negotiated with ease by highway construction crews.
I may have found the answer. The original highway, now the westbound lanes, swung to the north and passed through Ernfold. The eastbound lanes came later. Those lanes took a more direct path, avoiding further traffic through a residential area while reducing construction costs.
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Down along the border between México and the United States, a common geographic formation created dueling companion instances with the more impressive one located in México.
I measured the Interstate 8 median strip east of San Diego, California at 1.5 miles (2.4 km). The distance between opposing lanes of Mexican Federal Highway 2D as it stretched from Tijuana to Mexicali came in at around 2.6 miles (4.2 km). That was the same as the Canadian example, once again within the margin of error of eyeball precision. I am sure one or the other occurrence could claim the crown with improved criteria and measurement.
One other United States location often surfaced in discussions, a stretch of Interstate 24 outside of Monteagle, Tennessee (map). I measured the width of the median at 1.7 miles (2.7 km), the same as Pendleton, so I’d call these a tie for the greatest US distance unless someone finds a better location.
Sometimes examples from the United Kingdom surfaced. They did not compare favorably.
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The M6 at Shap near Junction 39 received frequent attention. One source noted that " Junction 39 is the highest junction on the M6 being less than half a mile from Shap summit, the highest point on the M6." That would seem promising. I measured it at 0.2 miles (0.32 km) which was even as much as Brazil’s Monumental Axis. Others suggested the A611 at Annesley (map). The central reservation appeared to have a similar width. They were both noteworthy for the UK although neither contended for the world title.
Canadian and Mexican examples, both with 2.6 mile (4.2 km) median strips, were the best that I could find during my cursory search. Certainly we can do better.