The photo above was posted by DDOT to its Tumblr feed earlier this month. It shows M St. in Georgetown at some undetermined date. Surely we can date it!
First off, the block shown here is the 3100 block of M St., just east of Wisconsin Ave. (you can just make out the edge of the Riggs bank on the far left of the photo). Towards the end of the block you can see the notable tall building that now houses Steve Madden.
Just starting from a guesstimate, based upon the cars, GM will say this is probably in the 1930s. The streetcar at the center is a PCC streetcar, which was built starting in the 30s, so that would be consistent.
Where the Urban Outfitters is now you can see the old Woolworth’s. But that’s not much help in dating it because that opened in 1922 (and closed in 1980).
The fact that there were still buildings standing between the Scott’s building and the Riggs bank doesn’t help since there were around in the 1950s.
The large neon sign for an optician is for J. Clinton Tribby Co., which was there long before the 1930s. So that’s not much help either.
But moving left down the street from Tribby’s it starts to suggest a date in the 1940s not the 1930s.
To the immediate left of Tribby’s is a shop called Adeline Shop. It sold women’s clothing. For what it’s worth, GM couldn’t find an advertisement listing this address until 1942 (but he only found one earlier other mention of the company in the 1920s).
Interestingly, the store where Old Glory is now was back then Scott’s Smart Shoes. While the font of the sign suggests an earlier Art Deco vintage, the earliest GM can find an ad referencing this store at all is 1947. And it appears that the company may have only used the “Scott’s Smart Shoes” for a few years around 1945. By the 1947 ad it was simply “Scott’s” (actually it was “Felser Scott’s”, which appears to have been a merger of two companies).
So maybe that’s getting us closer? Can any old car experts weigh in? Are there mid 1940s cars on the road?
The Georgetown Metropolitan 
Sorry, I don’t use social media accounts, so I can’t make an official reply. The M Street traffic jam photo can be dated at 1937 or later. The streetcars in the photo are the famous “Presidential” PCC cars which were introduced in Washington in 1937, first on the Georgia Avenue line.
City directories for 1940, 1943, 1948 and 1956 provide some clues, and suggest the picture is from the mid to late 1940s. Woolworth’s in the photo is in its present location at 3111 M Street; it moved there from 3125-27 M Street between 1940 and 1943. Scott’s shoes was not in the 1943 directory but was listed in 1948 and 1956. The Adeline Shop was in the old Woolworth’s space at 3125 M Street in 1943-56. The store to the left (west) of the Adeline Shop is a shoe store. From 1930-48, that space was occupied by the Schiff Brothers Shoe Store. In 1956, Edwards Shoe Store is listed. The first letter on the sign clearly is an E, but the first word appears too short to be “Edwards.”
So, the photo is from no earlier than 1943, and may be from the period between 1948 to the early 1950s if the store with the “E” sign is Edwards Shoes. The cars suggest no later than the early 1950s.