It is a custom design based on Erbar.

Erbar was a face from Bauer by Jakob Erbar in the 1920s. Only a few display weights of that have been digitized and made commercially available, sadly.

But if you want to match the Berlin city street signs, you are in luck!

Verena Gerlach and Ole Schäfer digitized this (and the East Berlin street signs) for the FF City Street Type family. To get this style, you should use FF Cst Berlin West Regular or Medium, I guess.

Actually, this is not a classic, because this looks like one of the new signs.

The original Berlin street signs were painstakingly digitized by type archaeologist Verena Gerlach and Ole Schäfer for FontFont as FF City Street Type in bothe East and West Berlin variants, before they started changing these wonderful signs into the somewhat less wonderful signs they have now.

I think the modern signs might use FF Transit, the typeface Erik Spiekermann and MetaDesign specified for the Berlin subway system. Is is available in three versions, the first one being FF Transit Print for use in print. The two other versions are intended for wayfinding -- FF Transit Front for front lit signs, and FF Transit Back which compensates the weight change of the characters in backlit signs so the weights of both front lit type and back lit type appear identical.

Hehehe, that'll teach me to write half a chapter when a simple answer could suffice.

To avoid any misunderstandings -- I am not correcting Dan in any way, it just took me longer to write my reply, which is why it ended up after Dan's. ;^)

Just checked FF Transit -- it is not, though it's might close except for the R and ß ligature.

So the modern signs use Transit, and the picture shows an older one in Cst Berlin West?

At least Dan is correct with FF Cst Berlin West -- check out the rounded lc. 'b' and the wonderful eszet (and tz!) ligatures. It has a marvellous 'period' feel, although I'm not 100% sure which period :-)