Measuring urban vitality: how do grand Paris express shape real estate markets ?[Mesurer la vitalité urbaine : comment le Grand Paris Express façonne-t-il les marchés immobiliers ?]

Territorial development extends beyond major cities to small and medium-sized towns, offering real estate and infrastructure opportunities. Urban expansion and metropolization raise concerns about accessibility to essential functions. The "15-minute city" concept is relevant, aiming to enhance proximity to work, housing, services, transport, and education. More generally, we use the concept of urban vitality to analyse the effect of new transport infrastructure in areas with different characteristics, but all under the influence of Paris. This study examines the impact of line 18 of the Grand Paris Express on local, social, and economic dynamics. To measure urban vitality, 1,686 urban and peri-urban areas along the line were analysed using 60 key variables from 200 databases, covering six functions: living, working, shopping, being in good health, learning and enjoying. Through principal component analysis and hierarchical classification, four territorial clusters were identified, reflecting differences in housing, density, amenities, and economic activity. New stations will serve diverse areas, offering insights into line 18's potential impact. Two areas were examined in detail: the Paris-Saclayeducational and research hub, a specialized area known for its innovation potential, and the Saint-Quentin Est station area, characterized by a mix of land uses with residential, industrial and tertiary spaces. The findings provide a territorial diagnostic to help real estate stakeholders identify investment opportunities, to assess project feasibility, and to anticipate market trends. The urban vitality clustering serves as a decision- making tool, offering a comprehensive view of local dynamics and supporting targeted development strategies aligned with sustainable urban planning objectives.