Beyond the Townhall: Spatial Anchoring and LLM Agents for Scalable Participatory Urban Planning

LLMs are increasingly explored for public participation, but less is known about how they should be designed around the information citizens encounter beforehand. In an experiment (N=200) on participatory urban planning, participants learned about a sustainability project adapted from a real street redesign, through text and images or narrated 360-degree video with spatial audio and spatial anchoring; they subsequently engaged with factual and reflective LLM personas. The immersive condition improved recall and changed what participants said to the agents: they named beneficiaries more often, asked directionally more project-related questions, and provided more positive feedback. We contribute (1) a conceptualization of LLMs as dialogue partners complementing a shared information baseline, (2) causal evidence that this baseline shapes those conversations, (3) an account of how citizens use and judge factual versus reflective agent roles, and (4) a method for cities to test information packages and agent configurations before public release.