MotionLM: Multi-Agent Motion Forecasting as Language Modeling

Abstract

Reliable forecasting of the future behavior of road agents is a critical component to safe planning in autonomous vehicles. Here, we represent continuous trajectories as sequences of discrete motion tokens and cast multi-agent motion prediction as a language modeling task over this domain. Our model, MotionLM, provides several advantages: First, it does not require anchors or explicit latent variable optimization to learn multimodal distributions. Instead, we leverage a single standard language modeling objective, maximizing the average log probability over sequence tokens. Second, our approach bypasses post-hoc interaction heuristics where individual agent trajectory generation is conducted prior to interactive scoring. Instead, MotionLM produces joint distributions over interactive agent futures in a single autoregressive decoding process. In addition, the model's sequential factorization enables temporally causal conditional rollouts. The proposed approach establishes new state-of-the-art performance for multi-agent motion prediction on the Waymo Open Motion Dataset, ranking 1st on the interactive challenge leaderboard.

Overview

MotionLM autoregressively generates sequences of discrete tokens for a set of agents to produce interactive trajectory forecasts. At each timestep, a token is sampled for each agent from a finite vocabulary and appended to the global sequence.

Bypassing geometric anchors and latent variable optimization, multimodal distributions emerge solely via per-step sampling. Meanwhile, the training objective is kept simple with minimal assumptions — just next-token prediction.

The resulting model can perform marginal, joint, and conditional forecasting. MotionLM establishes new state-of-the-art performance on both the Waymo Open Motion Dataset motion prediction and interaction prediction benchmarks.

Examples

Marginal vs. Joint

Attention-based interactive modeling during decoding allows for scene-level consistency. While marginal (independent per agent) predictions may lead to unrealistic overlap (left), joint predictions exhibit appropriate reactions across agents (right).

Marginal	Joint

Marginal vs. Conditional

When conditioning on a query agent trajectory (magenta), the predicted agent trajectory (cyan) can appropriately respond.

Marginal	Conditional