First ever signed duplex?

[bugo]

What was the first ever highway duplex to be signed?  Was it before 1900?

[DaBigE]

Couldn't really be before 1910, as most highways weren't numbered (at least officially). This triplex is estimated to be 1930-ish

[agentsteel53]

I've seen a c. 1920 photo of the National Old Trails Road and the Colorado to Gulf Highway on one guide sign at the top of Raton Pass.

did Wisconsin have multiplexes in 1918?

[Alps]

New England routes were signed well before the 1920s, though after the 1900s, and definitely had concurrencies signed (I forget the link, but it showed two of them painted on a pole, one above the other). Of course, you could have two named trails running together, back in the 19th century, but "signed" requires some sort of trailblazer for them. (Oh, hey, trail... trailblazer... hey!)

[Urban Prairie Schooner]

If pre-US highway era auto trails count for the purposes of this question, it was very common for up to six or seven (or more) different auto trails to share the same route, particularly in the Western states where there are fewer intercity highways overall. Since the establishment of the Lincoln Highway in 1913 was the catalyst for the growth of the auto trail movement, then certainly auto trail duplexes (and then some) would have been common in the period from around 1915 to the end of the auto trail era per the formation of the US highway system in 1926.  Granted, auto trails were not as consistently and systematically signed in the same manner as "public" state or US routes, so that detracts from this hypothesis somewhat.

The first public route duplex was probably in a state that developed a signed state highway system at an early date, possibly Wisconsin or Michigan.

[NE2]

Napoleonic France had signed routes, but I don't know if there were any overlaps.

[english si]

Routes.wikia.com has details on the imperial routes. Don't know if they signed common routes then, and it's not as if there was photographs or anything.

[triplemultiplex]

did Wisconsin have multiplexes in 1918?

Most assuredly.

[NE2]

A few old images of overlaps (Indiana, Wisconsin, North Carolina) beginning here: https://www.aaroads.com/forum/index.php?topic=207.msg250724#msg250724

[TheStranger]

Not sure what's California's first signed duplex...17/101 in San Jose?  40/50 in San Francisco (late 1930s) along Market Street?

[NE2]

Not sure what's California's first signed duplex...17/101 in San Jose?  40/50 in San Francisco (late 1930s) along Market Street?

66/99 Pasadena to San Berdoo and 40/99 Davis to Sacto are both original.

[TheStranger]

Not sure what's California's first signed duplex...17/101 in San Jose?  40/50 in San Francisco (late 1930s) along Market Street?

66/99 Pasadena to San Berdoo and 40/99 Davis to Sacto are both original.

40/99W existed on paper in 1932. Not sure if the US routes were signed before 1934 though.

[NE2]

Not sure if the US routes were signed before 1934 though.

They were by 1928.