Porticus Trajani (Portico di Trajano) «P. Vittore nella Reg. IX di Roma.»
The porticus is the most prevalent building typology within the Ichnographia Campus Martius. At least one porticus is found within every quadrant of the large plan, and cumulatively they add up as the most numerous building type within the Campus Martius, a portrayal which is both historically and archeologically sound. That is not to say, however, that each porticus Piranesi delineates has a historical basis. For example, the two Porticus Hadriani, the Porticus Horti Domitiae, at least one of the Porticus Neronianae, the Porticus praebentes umbraculum diei ab aesti, the Porticus Quirini, and the Porticus Trajani have no relationship with historical fact. Furthermore, many of those porticus that hold factual veracity, Piranesi dislocates within the large plan, and, overall, none of the porticus within the Ichnographia, with the exception of the Porticus Septorum Juliorum, express historically correct designs.
Even though Piranesi employs a different imaginative arrangement for each porticus delineated within the Ichnographia, he nonetheless exhibits an overall consistency within a discernible set of design patterns. Each porticus contains a profusion of colonnades. The oldest porticus are simply linear in plan, and those along major axes such as the Via Lati and the Equiria are linear in the extreme. The Porticus Horti Domitiae and the Porticus Europae incorporate circular motifs, and the two Porticus Neronianae are cruciform in plan. Finally, the porticus of Hadrian and the later emperors are very hybrid in that they incorporate any number of colonnade plan motifs, as well as contain an array of enclosed spaces.