There is an argument how many walls does a Rshus Harabim need to have to make it a Rshus Hayachid (Deoraisa). R' Yehuda says that one needs two walls, while the Chachamim said one needs three.
The Gemara (there) says that a Sratya (a through street going through the city) and a Playta (a city square) are Rshus Harabim. However, according to R' Yehuda, all Sratyas and Platyas are surrounded by walls on two sides.
Notice the Sratya and Platya surrounded on two sides by "black" area.