In the pre-automotive era of cities, traces of the concept of a hierarchy of streets in a network appear in Greek and subsequent Roman town plans. The main feature of their classification is their size. In Roman cities, such as Pompeii, major thoroughfares (e.g. the decumanus) had a width of 12.2 m, secondary streets (e.g. the cardo) 6 m and tertiary streets (e.g. vicinae) measured 4.5 meters. The first allowed for two way cart traffic, the second generally only one, while the third only loaded animals. Narrower streets that could only accommodate pedestrians were also present in both Greek and Roman cities. Thus the restriction on connections between major streets on particular modes (carts and chariots) was the effect of the width of the street itself and not the lack of linkage. This method is akin to the contemporary concept of filtered permeability.