The grid layout PAGE PAGE PAGE

Plan D. Busson and V. Charlanne. Cliché IGN Photothèque Nationale - 1999.
  Secondary orientations
Secondary orientations have been found outside the town, probably corresponding to roads leading to Lutetia in order to have access to the bridge. It is difficult to know whether, like the Melun road, they were part of the original urban plan. It is likely that a section of the rue de Vaugirard was a Roman road, perhaps one that ran along the edge of town and was the end-point for several decumani.
Another significant diverging road can be seen in the Rue Saint-Jacques cardo which deviates before returning to its original path. Several roads and houses found nearby were built around this deviation-this is probably because it was a link between the new axis laid out by Lutetia's founders and an older, apparently protohistoric road that connected Lutetia to the Gallic oppidum of Genabum.

We can see that this deviation is located right at the edge of Lutetia (after this point the road becomes a highway). However, it is possible that the deviation represents an adaptation of the road to an aspect of the terrain. This type of geographical adaptation can be used to explain other deviations from the grid system. This is the case with the Île de la Cité and the small district on the Saint-Séverin hillock. Their small surface areas made it difficult to incorporate them into the grid plan, and their location in flooding zones meant that they could not be built up systematically. The most we can say is that the cardo maximus probably connected with a series of small perpendicular streets that led to housing areas.