Many streets in Pompeii were not wide enough for two-way traffic. Some researchers believe that some of the streets may have been permanently one-way, although markers indicating a traffic direction have not as yet been identified. Archaeologists have identified predominant directions from some of the streets by looking at the patterns of the ruts.
It is also possible that the one-way direction of some streets was 'as needed', with consistent movement of carts assisted by the clanging of loud bells, screaming merchants and small boys running around leading traffic.
Sources
For more on the archaeology of Pompeii, see Pompeii: Buried in Ashes. Also see the Walking Tour of the House of the Faun.
Beard, Mary. 2008. The Fires of Vesuvius: Pompeii Lost and Found. Harvard University Press, Cambridge.
