From a biodiversity point of view, "twinning" may be of interest to the latter on two of the areas considered. By creating interstitial spaces, "twinning" can, in regions of ordinary habitats, contribute to the reconstitution of biodiversity-friendly environments to the extent that the species encountered in these environments (flora, butterfly, reptile) do not are not significantly different from those present nearby. In terms of ecological continuity, "twinning" limits the increase in the degree of fragmentation induced by the realization of the second infrastructure. It also promotes partial connectivity for some open-sided species by acting as a longitudinal corridor. On the other hand, if ecological transparency works make it possible to temporarily restore connectivity to the right of one or the other of the twinned infrastructures, these constitute a physical filter for the dissemination of certain species because of the lack of coherence. amenities.
The analysis of the landscape and land use on the studied sites shows that the implementation of a second infrastructure in twinning leads to less changes in the landscape structure than those which occurred during the construction of the first, because the redevelopments land are more limits. The landscape insertion of the twinned infrastructures is different according to the more or less important technical constraints (relief, radii of curvature) to which their trace must adapt; hence, variations in distance between the infrastructures then determine contrasting situations for the use of interstitial spaces. These notable direct effects, with the creation of new landscapes along the trace of infrastructures in the case of development of semi-natural habitats (wastelands, hedges, thickets), contrast with the weak influence of infrastructure on socio-demographic trends. territories studied.
From the societal point of view, we note, in the first place, the lack of spontaneous and specific representation of "twinning" with the different actors of the territory. This notion does not make sense or is not associated with the two infrastructures studied and the very principle of "twinning" is only rarely understood. It has not been identified a territorial anchorage or a specific relationship of the twinning to the crossed territories. Similarly, the mobilizations do not depend so much on the specific characteristics or the principle of the "twinning" of infrastructures that the context (social, economic, historical and political) of the territory concerns or more exactly the representation that these actors have of the territory. We also observed a variation in the perception and perception of residents' nuisances according to the interlocutors (installation times in particular) and the scales of analysis (nature and the role of the more or less detailed service of the territory). But it seems to emerge a relative acceptance of "twinning" among residents on the medium and long term. Finally, the lack of synergy between the two transport infrastructures in terms of project management is blatant and increases with the time lag between projects.
Through a transversal approach, the comparison of the elements resulting from the various thematic advantages / disadvantages analyzes highlights the complexity of the implementation of the twinning. Each of the themes has established specific findings, but some issues are however shared: the need for coordination between building owners, the appropriate management of interstitial spaces, the issue of fragmentation of space, the need to adapt the twinning with each territory crosses, relativity of the advantages / disadvantages of twinning according to the scales of reflection and the posture of the actors concerned. The synthesis of the cross-cutting elements presented illustrates the extent to which the question of "twinning" and its effects and perceptions is multiple and complex. It goes beyond the simple technicist approach of infrastructure developer. The analysis of the impacts alone can not make a decision on its realization, as the relations of scale, the relations between owners and decision makers and the relations with the inhabitants of the territory condition its implementation and benefits potential that a twinning can bring to the territories.