HighJax: Highway Driving environment for Reinforcement Learning research

PPO agent learning to drive on a 4-lane highway

HighJax is an autonomous driving environment for Reinforcement Learning research. It's a JAX implementation of the HighwayEnv. HighJax provides a fully JIT-compilable and vectorizable highway driving simulation.

Besides being much faster than the original, it provides Octane, a Rust-based TUI for examining your experiment runs. Octane provides an interface for defining behaviors and then measuring how much each policy exhibits them.

HighJax was produced as part of our research project about BXRL: Behavior-Explainable Reinforcement Learning.

Installation

pip install highjax # Minimal installation
pip install "highjax[cuda12]"# Including GPU support
pip install "highjax[trainer]"# Including PPO implementation
pip install "highjax[cuda12,trainer]"# Including both

Quick Start

importjaximporthighjaxenv, params=highjax.make('highjax-v0')
key=jax.random.PRNGKey(0)
obs, state=env.reset(key, params)
obs, state, reward, done, info=env.step(key, state, 1, params) # IDLE

Using with JAX RL Libraries

HighJax follows the gymnax API, so it works with JAX RL frameworks that expect gymnax-style environments:

• PureJaxRL — drop-in gymnax replacement (no PureJaxRL install needed), see examples/use_purejaxrl.py
• Stoix — via stoa gymnax adapter, see examples/use_stoix.py
• Rejax — pass env object directly, see examples/use_rejax.py

Training

Train a PPO agent via the CLI:

highjax-trainer train

Key options:

Flag	Default	Description
--n-epochs / -e	300	Training epochs
--n-es	400	Parallel episodes per epoch
--n-ts	40	Timesteps per episode
--seed / -s	0	Random seed
--actor-lr	3e-4	Actor learning rate
--critic-lr	3e-3	Critic learning rate
--n-npcs	50	NPC vehicles
--no-trek	—	Disable trek recording
--n-sample-es	1	Episodes to sample per epoch for trek
--trek-path	auto	Custom trek directory path
--discount	0.95	Discount factor (gamma)
--n-lanes	4	Number of highway lanes

Training automatically records episode data to ~/.highjax/t/ for browsing with Octane (the TUI). Use --no-trek to disable.

Here's a snazzy one-liner that will let you explore the results of the current experiment run using VisiData:

pip install visidata
vd "$(ls -d ~/.highjax/t/2*/ | tail -1)"/epochia.pq

Use the following command line to produce similar results as seen in Figure 2 of the paper:

highjax-trainer train --n-es 128 --n-ts 400 --n-epochs 300 --target-kld 0.0005

Octane (Episode Browser)

This repo also includes Octane, which is a Rust-based TUI for browsing HighJax experiments.

Installation

sudo apt-get install build-essential # C toolchain (needed by Rust)
sudo apt-get install ffmpeg # Needed for `octane animate`
git clone https://github.com/HumanCompatibleAI/HighJax # Clone this repocd HighJax
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh # Install Rustsource"$HOME/.cargo/env"cd octane && cargo build --release # Build Octanealias octane="$(readlink -f octane/target/release/octane)"

The binary will be at octane/target/release/octane.

Usage

After training, launch Octane to see all the experiments you ran with highjax-trainer:

octane

Figures

Use Octane to make figures for your paper:

octane draw -t ~/.highjax/t/2026-03-15_20-02-25_101327 --epoch 300 -e 0 --timestep 19 --theme light \
--zoom 1.8 --png ~/figure.png

Behavior crafting

Octane includes a behavior explorer for defining measurable policy properties. While watching an episode, press b to capture a scenario — mark which actions you want (positive weight) or don't want (negative weight) at that traffic state. Name it, and Octane saves the behavior to ~/.highjax/behaviors/. The next time you run highjax-trainer train, all discovered behaviors are evaluated every epoch and their scores are recorded as behavior.{name} columns in epochia.parquet.

Defining a behavior scenario in Octane

Press B (Shift-B) to open the full Behavior Explorer tab.

See the Octane docs for full details.

Documentation

Full documentation is in the docs/ folder:

• HighJax environment docs — state, observations, reward, NPCs, physics
• Octane TUI docs — episode browser, configuration, key bindings
• Coding conventions — naming, array indices, style

Examples

• examples/basic_usage.py — Create env, reset, step, print observations
• examples/train_ppo.py — Train a PPO agent and evaluate it
• examples/use_purejaxrl.py — PureJaxRL integration (vectorized scan loop)
• examples/use_stoix.py — Stoix integration (via stoa gymnax adapter)
• examples/use_rejax.py — Rejax integration (JIT-compiled training, vmapped seeds)

Citation

If you use HighJax in your research, please cite:

@article{rachum2026bxrl,
title={BXRL: Behavior-Explainable Reinforcement Learning},
author={Ram Rachum and Yotam Amitai and Yonatan Nakar and Reuth Mirsky and Cameron Allen},
year={2026},
eprint={2603.23738},
archivePrefix={arXiv},
primaryClass={cs.LG},
url={https://arxiv.org/abs/2603.23738},
}