There is a sign at each entrance with a structure that has a camera over it that scan each car.

If you go on this site: http://www.atac.roma.it/menu.asp?Cod...adre=1&p=1&i=9

the maps for all the restricted zones are there. Be careful mainly to "Settore C"; the yellow-line is the limit of the zone and the red circle is where you cannot pass if you don't want to get scanned. The road right next to the river is excluded from the zone and you can use it.