According to [http://www.laleggepertutti.it/94376_autostop-chiedere-un-passaggio-e-illegale this legal article (Italian)], this does not apply to ''tangenziale'', bypasses, unless they are ''autostrade'' or other high-velocity roads. All in all, it's less likely that you will encounter problems if you're not Italian -- and, if you speak Italian, don't reveal this to police (or the armed forces, ''carabinieri'').
Border crossings that are on the motorways (such as the Grand Saint-Bernard at the Swiss border) might be considered by the police as part of the highway. [[User:Olche Maith|Nina]] was
hitchhing with a nice truck driver who was trying to help her get her next ride at the border, but the policeman did not allow her to continue hitchhiking (even not to stay in the truck with the same driver!) and instead drove her to the bus stop and gave her money (much more than required for the bus). Others might not be so generous though.
Edit: (TommytheCommie) We had no success trying to hitchhike from the toll bridges at the entrance to highways (autostrade). Everybody in Italy knows it's illegal to hitchhike on the highway. The police moved us back to the roundabout before the toll bridge, which was fine, but nobody was stopping in either position. So instead we hitchhiked on minor roads (named SS-9/10/etc.) and never waited longer than thirty minutes once we found a good spot. We hitchhiked down from Alessandria to Bologna on the Via Emilia (SS-9), getting short rides of 20 or 50 km at a time, but never waiting long.