Mode 0: A New 3GPP V2X Resource Allocation Category for Roadside Computing Unit-Assisted Safety Communication

The 3GPP Vehicle-to-Everything (V2X) resource allocation framework defines two entity classes — the base station and the vehicle User Equipment (UE) — and four modes across LTE and NR generations. We demonstrate that this binary taxonomy is structurally incomplete. Base station-led scheduling saturates at high-density traffic nodes, producing latency-tail failures even when mean Packet Delivery Ratio (PDR) meets the service-class target. UE autonomy is categorically incapable of pre-emergence warning for occluded participants, and insufficient for large-scope cascading environmental hazards. We propose Mode 0, a new 3GPP V2X category whose defining entity is the Roadside Computing Unit (RCU) — an infrastructure ensemble integrating elevated sensing (Seeing), sidelink communication (Speaking), and local computational evaluation (Thinking) under traffic-management ownership. Mode 0 defines a subfamily spectrum: Mode 0a (all-passive UEs, the guaranteed minimum) through Mode 0c (all-active UEs, the optimal target). Convergent deployment evidence across Chinese national standards, operator infrastructure (China Unicom RS-MEC), and European and US C-V2X programs confirms both institutional sides converging on the roadside traffic node without a coordination standard. A fifteen-run Multi-Agent Proximal Policy Optimization (MAPPO) simulation programme validates the architectural family: Mode 0a in the shared-pool baseline sits at the analytical symmetric-Nash coordination floor, provably unbreakable by shared-actor architecture; Mode 0c with demand separation achieves strict Pareto improvement for both classes (M0 PDR 0.999, M1 PDR 0.998), and lifts the worst-TTI PDR from near-zero to 0.601 — the only configuration satisfying the 95th-percentile latency requirement as a structural property. We call for a 3GPP study item on Mode 0 within the NR-V2X sidelink enhancement work programme.