Braking Scenario: Hard Stop Evaluation (Deterministic)

This document provides a technical guide to the Lead Vehicle Hard Braking scenario, including its architectural design, common CARLA-specific pitfalls, and the robust fixes implemented to ensure 100% repeatability.

🏗️ 1. Scenario Architecture

The Braking scenario consists of two main actors on a multi-lane road (default: Town10HD_Opt):

Ego Vehicle: The vehicle under test, tracking a set of target coordinates or following a lead vehicle.
Lead Vehicle: An NPC spawn 25m ahead on the same lane.

Success Metric: The Ego ADNC stack must detect the lead vehicle's emergency deceleration and brake safely without collision.

🏎️ 2. Critical Spawning Fixes (Lessons Learned)

During implementation, we identified several subtle CARLA behaviors that led to RuntimeError: Spot occupied failures. These are now documented for all future scenario developers.

🛑 Issue A: Stale Physics (Synchronous Mode Delay)

Problem: When spawning a new actor in Synchronous Mode, its location (get_location()) often defaults to stale values (like 0,0,0) until the world's physics engine has processed at least one frame.
Impact: Calculating NPC spawn points based on the Ego's position immediately after spawning resulted in NPCs appearing at the map origin instead of ahead of the Ego.
Fix: Added an explicit world.tick() call in the core orchestrator immediately after the Ego is spawned and before the scenario's setup() method is called. This "settles" the Ego and guarantees accurate coordinate data.

🛑 Issue B: Ground-Mesh Clipping & Bounding Box Overlap

Problem: CARLA waypoints sit exactly on the road surface (Z = 0.0). Spawning a vehicle's center-point at this height causes its collision box (chassis and tires) to overlap with the road mesh.
Impact: CARLA identifies this overlap as an "obstacle" and fails the spawn with a "Spot occupied" error.
Fix: Implemented a mandatory 0.5m Z-offset (lift) for all NPC spawn transforms. NPCs are spawned slightly in the air and "dropped" onto the road, which is the industry-standard way to ensure collision-free actor placement in CARLA.

🛑 Issue C: Deterministic "Spawn-and-Move" Pattern

Problem: Directly spawning an actor at an absolute intersection coordinate (P2 from our showcase) is fragile and frequently fails due to microscopic geometry conflicts in Town10.
Fix: Developed a two-stage Spawn-and-Move pattern:
1. Spawn the Ego at a "safe" map spawn index (provided by map.get_spawn_points()).
2. Immediately translate it to the target scenario coordinates using set_transform().
- Result: set_transform is significantly more lenient than try_spawn_actor, ensuring we never fail to reach our starting intersection.

🧪 3. Parametric Tuning via CLI

The orchestrator now supports dynamic parameter injection for the braking scenario:

./run.bat braking --params "BRAKE_FRAME=120,LEAD_DISTANCE_M=30"

BRAKE_FRAME: The exact simulation frame where the lead vehicle disengages autopilot and applies a brake=1.0 override.
LEAD_DISTANCE_M: The initial separation between Ego and Lead vehicle.

Created: March 2026 | Intel Repository — ADAS Development

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