This is not really a warning, but rather an enigma to us.
Well, it used to be thanks to Elena_N who unravelled this mystery for us:
It is a sign which is meant to inform other traffic users that this is an area, where visually impaired people are likely to cross the street.
Very mild warning. . .at several churches and cathedrals, there are Kassas outside. These are usually for Russians only. . .a quick peek at the signs on the booth will confirm, but often tickets for foreign visitors are sold inside.
Among the 2000 streets of the city, some are called : linia (line), proezd (passage), alleïa (alley), protok (river), doroga (road), chosse (pavement), boulvar (boulevard). Some examples : Smolnaïa naberejnaïa (Smolny embankment) - 2,6km, Smolnovo oulitsa (Smolny street) - 600m, Smolny prospekt (Smolny avenue) - 300m, Smolnovo alleïa (Smolny alley) - 200m, Smolnovo proezd (Smolny passage) - 300m. These different streets are situated in the same district, no far from the Smolny monastery. Morskaïa naberejnaïa (Maritime embankment), situated close to the Baltic sea on the Vassilievski island, is to a distance of more than 10km of the avenue of the same name, Morskoï prospekt, that is on its turn very far from Morskaïa oulitsa (Maritime street). In more, there is Bolchaïa Morskaïa oulitsa (Big Maritime street) and Malaïa Morskaïa oulitsa (Small Maritime street). The two last are situated in level center, between the Saint-Izaac place and the main street of the city, Nevski perspective. There is two Rues Mikhaïlovskaïa (of Michel), one - in the center, close to the museum of the art Russian, the other is situated in the Primorski precinct ; there are an alley, Mikhaïlovski pereoulok, and a Rue Mikhaïlova (of Mikhaïlov). has two avenues that carry the name of Bolchoï prospekt (Big avenue). It requires the precision : Bolchoï V.o prospekt. (Vassilievski ostrov - the island of Vassili) or Bolchoï P.s prospect. (P.s. - Petrogradskaïa storona - side of Petrograd). The names of some streets include a number: 1st - 10th Sovetskaïa oulitsa, 1st - 29th Linia V.o. The city, situated between more of 90 channels and rivers, has the streets - embankments that are called of the river name: naberejnaïa Reki Moïki (embankment of the River Moïka), naberejnaïa Reki Smolenki (embankment of the River Smolenka). The arms of the Neva are called : the Big Neva, the Small Neva and the Big Nevka (small Neva) that divides in the Middle Nevka and the Small Nevka. The embankments of the Neva and its arms carry the clean names.
Luckely it's going better and better with Russia and it's economy. However, take into mind that not many Russians speak English or any other language besides their own. To be able to at least read the signs (understanding may still be difiicult) try to learn the Cyrillic alfabet. This way you won't get lost in the metro and know the ÐÅÑÒÎÐÀÍ actually is RESTORAN (restaurant) and ÀÏÒÅÊÀ is not an antique-shop, but a drugstore (apteka = apotheque).