UX: Audio-First, Minimal Visual¶

Two modes. On-track: the co-pilot (audio + minimal HUD). Off-track: the race engineer (full dashboard).

The mode boundary is not a tab the driver picks — it switches automatically based on speed. The driver never has to think about which screen they're on.

Design Principles¶

These five principles override every UX decision. When a feature seems valuable but conflicts with one, the principle wins.

1. Audio-first, peripheral vision second¶

A driver at 130 mph on the back straight has zero capacity for reading text. The co-pilot's primary channel is TTS through Pixel Earbuds; the screen is a peripheral-vision signal at most. We test every visual element by asking: can a driver read this without taking their eyes off the apex marker? If not, it's audio.

Reference: ADR-013 (Frontend visualizes, backend reasons)

2. Silence is coaching¶

The arbiter (ADR-002) holds 78% of generated cues. Saying nothing when the driver is on a clean line is a feature: it tells them they're doing it right, and it preserves attention for the cue that actually matters. We measure cue suppression rate, not cue volume.

3. Fail open, never silent¶

If the bridge dies, the LLM times out, the network drops, or earbuds disconnect — the system degrades to the next-tier coach (sonic_model → mock → silence) without the driver having to do anything. No error dialogs on track. The signal-light HUD turns red if grip is exceeded regardless of whether the AI is reasoning.

Reference: ADR-009 (Graceful degradation)

4. Confidence is part of the message¶

Coaching with high model confidence sounds different from low-confidence coaching. A confident cue is direct ("brake at the bridge"); a low-confidence cue is hedged ("you might want to brake earlier") and arrives quieter. Drivers learn to weigh advice by how it's phrased, not by a "AI confidence: 0.62" overlay they can't read.

Reference: ADR-001 (Confidence-annotated frame)

5. No number-chasing on track¶

We deliberately hide lap times, deltas, and split timers from the on-track HUD. Drivers who chase numbers stop driving the corner in front of them. Numbers belong in the paddock — that's why mode-switching is automatic and the on-track screen is two coloured bars, not a dashboard.

Implementation conventions (PWA)¶

The PWA at src/pwa/ is landscape-only and standardises a handful of interaction patterns. These supersede earlier per-page conventions.

Unified key contract¶

App.vue owns the global key handlers; pages only bind page-specific keys.

ESC      ─▶ router.back()         (every page except /, and only when no overlay is up)
ESC      ─▶ closePause()          (when the pause overlay is visible)
P        ─▶ togglePause()         (only on routes with meta.allowPause = true)

Page-local conventions, applied consistently where they're bound:

1-9      ─▶ switch tab            (CyberTabs-tabbed pages, Analysis Hall = 1-6)
◀ / ▶    ─▶ in-page cycle         (laps, list selection, …)
B / ⌫    ─▶ back / dismiss        (also: resume in PAUSE)
A / Enter─▶ confirm
T        ─▶ open agent trace      (HUD)
M        ─▶ trigger voice FAB     (HUD, Quest Log Ask Coach)
O        ─▶ toggle outlier filter (Analysis Hall)
Shift+D  ─▶ debug overlay         (HUD)

ESC is never rebound at the page level. Earlier per-page ESC handlers were a source of dead-end navigation when overlays stacked.

Touch targets ≥ 44 × 44¶

Every tappable element — toolbar selects, tab buttons, dialog actions, HUD trace button, voice FAB — meets the 44×44 minimum so that a gloved hand on a vibrating windshield mount still hits it. CyberButton, CyberCheckbox, CyberTabs enforce this by default.

Tabbed pages¶

Content-dense pages (Settings, Car Setup, Calibration, Hardware Detail, Pit Stall, Live Pit Wall, Quest Log, Coach Codex, End of Day, Pre-Brief, Trainer Card, Analysis Hall) use the CyberTabs widget from src/pwa/src/shared/ui/core/CyberTabs.vue. Tab state is owned by a per-page store (Pinia) so reopening the page restores the active tab. 1-9 switches tabs; ←/→ cycles in-page selection within the active tab where applicable.

Pixel 10 landscape, primary viewport¶

The PWA targets the Pixel 10 in landscape (≈ 2424 × 1080 px). The manifest declares display: 'fullscreen' and orientation: 'landscape'. Portrait viewports render a persistent portrait-warning overlay ("PLEASE ROTATE YOUR DEVICE") and block all interaction — landscape is load-bearing for the cockpit layout.

FullscreenToggle (mounted globally) persists the user's choice in localStorage.fullscreenPreferred and re-requests fullscreen on subsequent visits where a user-gesture window allows it. InstallPrompt captures Chrome's beforeinstallprompt, surfaces a sticky bottom banner, and respects a 7-day dismiss cooldown so it doesn't nag.

Transitions + audio cues¶

Route changes play a directional wipe (meta.wipe ∈ {right, left, up, down}) via TransitionWipe. Audio SFX (audioStore.playSfx) fires on every key event: cursor_select, cursor_move, cancel, transition_wipe, error_quiet. Reduced-motion (save-slot preference) drops both the wipe and the SFX.

User Journey — A Day at Sonoma¶

The whole UX is structured around a track-day's natural arc. Each stage maps to specific endpoints in src/pitwall/__main__.py.

flowchart LR
  classDef paddock fill:#1a3a52,stroke:#4a6e8a,color:#e0e0e0
  classDef track fill:#5d4a1a,stroke:#8a6e3a,color:#e0e0e0
  classDef trans fill:#3a3a3a,stroke:#6e6e6e,color:#e0e0e0

  ARRIVE[Arrive<br/>connect hardware]:::paddock
  BRIEF[Pre-brief<br/>set 1–3 goals]:::paddock
  OUT[Out-lap<br/>warm tires + system check]:::trans
  HOT[Hot laps<br/>co-pilot mode]:::track
  COOL[Cool-down lap]:::trans
  DEBRIEF[Paddock debrief<br/>review + next session]:::paddock

  ARRIVE --> BRIEF --> OUT --> HOT --> COOL --> DEBRIEF
  DEBRIEF -.->|next session| BRIEF

Stage	Duration	UX	API endpoint
Arrive	5 min	Pair earbuds; bridge auto-detects Racelogic + USB-CAN. Health check shows engine status, DuckDB ready, track loaded.	`GET /health`
Pre-brief	2–4 min	Driver picks 1–3 session goals (corner focus / lap-time delta / technique). System reads weather phase, surfaces danger zones, picks today's focus corners from prior driver profile. Goals are spoken aloud once before pulling out, then become coaching anchors during the session.	`GET /coach/brief`, `GET /track/weather`, `GET /track/danger_zones`, `GET /driver/<id>/profile`
Out-lap	1 lap (~2 min)	Co-pilot mode is on but only safety cues fire (P3). The driver hears no technique coaching while tires warm. Sonic-model grip tone is at 50% volume so the driver gets used to it.	`POST /analyze` (P3-only)
Hot laps	4–8 laps	Full co-pilot. Audio coaching gated by the arbiter; signal-light HUD in peripheral vision; the 4 audio layers (grip, brake, throttle, trail) plus pace notes. Cue volume scales with the current lap-time vs goal — closer to goal, less coaching.	`POST /analyze`, `POST /session/<sid>/frames`, `POST /session/<sid>/signals`
Cool-down	1 lap	All cues silenced except the corner-score chime. Driver hears "you went +0.4s, +0.1s, -0.2s, …" — pure summary, no advice. Lets the driver build their own mental model before the AI weighs in.	`POST /analyze` (chime-only)
Paddock debrief	5–15 min	Engineer dashboard. AI debrief is the lead, then lap table, sector trends, friction circle, per-corner grades. The driver can ask the coach a follow-up: "Why did I lose time at T7?" — answered with citation back to the telemetry burst. Goals from pre-brief are scored: did you hit them?	`POST /coach/debrief`, `GET /session/<sid>/scorecard`, `GET /session/<sid>/lap_time_table`, `GET /session/<sid>/corners`, `GET /session/<sid>/ideal_lap`, `POST /score`
Iterate	—	Debrief feeds the next pre-brief: today's focus corners come from yesterday's weakness. The driver sees their evolution chart over the season.	`GET /driver/<id>/evolution`

Every coaching moment is grounded in a previous moment. Out-lap warms into hot laps; hot laps feed the debrief; the debrief seeds tomorrow's pre-brief.

Validated Cue Distribution (from Sonoma Simulator)¶

Running the LSTM-driven sonic model v2 on 11,737 frames of real Sonoma telemetry:

Audio Layer	Frames Active	% of Session	Priority	When It Fires
Grip tone	6,475	55.2%	P0 (background)	Continuous — pitch tracks friction circle utilization
Brake approach	4,970	42.3%	P1-P2	Corner approaching, ascending pitch
Speed delta (LSTM v2)	~2,100	~18%	P1-P2	When actual speed deviates >5 km/h from LSTM prediction
Throttle cue	377	3.2%	P1	Past apex, driver not on throttle yet
Coast warning	354	3.0%	P1	Throttle <10%, brake <2 bar, speed >30 km/h
Trail brake	138	1.2%	P1	Brake >3 bar AND gLat >0.4G in a corner
Corner score chime	~55	~0.5%	P0	On exit of each corner (up/down/neutral chime)
Silence	2,545	21.7%	—	On straights when everything is normal

Key finding: The driver hears audio cues 78% of the time, with 22% silence. The grip tone is the continuous background layer — the driver habituates to it and notices changes in pitch. Active coaching (brake approach, speed delta, throttle) fires in ~25% of frames. This is the right balance — enough information without overwhelm.

On-Track: The Co-Pilot¶

The driver's cognitive capacity at 130 mph is fully consumed by driving. The UX must add information without adding cognitive load.

Audio (Primary Interface)¶

All coaching delivered via Pixel Earbuds TTS.

Message Type	Length	Example	Priority
Safety alert	1-2 words	"BRAKE!" / "Lift!" / "Car right!"	P3 — immediate
Reflexive cue	2-5 words	"Trail brake." / "Commit." / "Full send."	P2 — on straight
Technique	5-15 words	"Trail brake to the apex. Smooth release."	P2 — on straight
Strategy	10-25 words	"Turn 3: you braked 15m early vs AJ. Try holding to the 2-board."	P1 — queued

Delivery timing: The message arbiter holds non-safety messages until the car is on a straight (|gLat| < 0.3G). This prevents mid-corner distraction.

Message cadence: Maximum one message every 3 seconds. A lap at Sonoma is ~100 seconds. That means ~15 coaching opportunities per lap, but typically 3-5 are used. Silence is coaching too.

Coaching personas¶

The same coaching content can be delivered in five voices. Persona is a setting on the driver profile (changeable any time, no session restart) and only affects phrasing — the substance of every cue is identical across personas.

Persona	Voice	Example: late-apex T11 cue	Best for
T-Rod (default)	Rally pace-note, terse, environmental anchors	"Wait for the bump, trail to the third tire stack, all the road on exit."	Drivers who already know the track
Bentley	Classroom, technical	"Initiate trail-brake at the bump. Maintain pressure to the apex; ease off as steering peaks. Open the wheel on exit and unwind progressively."	First-timers, learning the technique
Drill Sergeant	Direct, urgent	"Brake! Trail it! Apex! Throttle!"	Drivers who under-commit
Calm Pro	Quiet, measured	"Settling for T11. Brake on the bump. Trail to the apex."	Drivers prone to over-driving
Buddy	Conversational, warm	"Okay, T11 here — wait for that bump to settle, ride the brake all the way to the apex, you got it."	Beginners, anxious drivers

Each persona pulls from the same Sonoma vocabulary (SYSTEM_PROMPT_LORE in sonoma.py — the bridge, the K-wall bend, Calamity Corner, the bump, the third tire stack). Personas don't invent track terms; they just rephrase. This keeps the safety-critical content stable while letting the register match the driver.

Skill level (beginner | intermediate | pro) is orthogonal: a beginner gets longer, more explanatory cues in any persona; a pro gets terser cues in any persona. The matrix is 5 personas × 3 skill levels = 15 phrasing modes, all driven by the same underlying coach decisions.

Signal Light HUD (Secondary Interface)¶

The Pixel 10 screen shows minimal visual information. No graphs. No numbers. No text.

┌─────────────────────────┐
│                         │
│    ┌───┐       ┌───┐    │
│    │   │       │   │    │
│    │   │       │   │    │
│    │ G │       │ R │    │
│    │ R │       │ E │    │
│    │ E │       │ D │    │
│    │ E │       │   │    │
│    │ N │       │   │    │
│    │   │       │   │    │
│    └───┘       └───┘    │
│                         │
│   GRIP OK    OVER LIMIT │
│                         │
└─────────────────────────┘

Left bar (green): Grip available. Height = percentage of friction circle unused. Full bar = car is well within limits. Shrinking = approaching the limit.

Right bar (red): Over-limit. Height = how far beyond the grip circle. Appears only when sqrt(gLat^2 + gLong^2) > max_G * 0.95. Growing = sliding more.

Why this works: The driver doesn't need to read it. In peripheral vision, green = OK, red appearing = back off. One glance, zero cognitive parsing.

What the HUD Does NOT Show On-Track¶

Lap times (distracting — driver focuses on chasing numbers instead of technique)
Speed (driver can feel it)
RPM (driver can hear it)
Tire temps (driver can't act on it mid-corner)
Coaching text (that's what audio is for)
Dashboards, graphs, or charts of any kind

Off-Track: The Race Engineer¶

When the car enters the paddock (speed < 5 mph for >30 seconds), the system switches to engineer mode: a full analytical dashboard on the Pixel 10 screen.

graph TB
    subgraph Engineer Mode Dashboard
        LAP[Lap Times<br/>table + delta to best]
        TRACE[Speed Trace<br/>current vs best vs Gold Standard]
        CORNER[Corner Report Card<br/>per-corner scores]
        FRICTION[Friction Circle<br/>gLat vs gLong scatter]
        PROFILE[Driver Profile<br/>skill radar + trends]
        DEBRIEF[AI Debrief<br/>LitertCoach session analysis]
    end

Dashboard Panels¶

Session Goals¶

The pre-brief lets the driver set 1–3 session goals. Each goal is one of three shapes:

Goal kind	Example	How it's scored
Corner focus	"Carry more apex speed at T7"	Apex speed delta vs prior session for that corner
Lap-time delta	"Break 1:48 on a clean lap"	Best lap of session vs target
Technique	"Trail-brake every corner entry"	% of corner entries with brake>3 bar AND

The dashboard's first panel shows each goal with a check (✓ achieved), partial (◐ improved but not hit), or miss (✗) — and the delta in human terms ("apex speed at T7 went 81 → 84 km/h, +3 km/h"). Goals carry forward: if a goal misses, it's pre-selected for the next pre-brief.

The on-track audio coach knows the goals too. If "carry more apex speed at T7" is a goal, the coach pre-empts T7 cues in earlier corners ("setup for T7 — exit T6 at full throttle") and stays quieter elsewhere. Goals shape attention.

Inspired by trustable-ai-codelab's SessionGoal API; pitwall ties scoring to the existing per-corner aggregates already exposed at GET /session/<sid>/corners.

Lap Times¶

Table of all laps with delta to personal best and Gold Standard (AJ).

Speed Trace Overlay¶

Three overlapping speed traces by track distance: - Current session best (blue) - Personal all-time best (green) - Gold Standard / AJ (gold)

Shows exactly where the driver is faster/slower and by how much.

Corner Report Card¶

Per-corner scoring:

Corner	Entry Speed	Min Speed	Exit Speed	Trail Brake	Time	vs AJ
Turn 3	78 mph (B)	52 mph (C)	68 mph (B)	15% (A)	4.2s	+0.4s

Grades: A (within 5% of AJ), B (within 15%), C (within 25%), D (>25% gap).

Friction Circle¶

Live gLat vs gLong scatter plot from the session. Shows grip envelope utilization.

Driver Profile (Event-Sourced)¶

Skill radar chart computed from DuckDB session data. Dimensions: Braking, Trail Braking, Corner Speed, Throttle Application, Consistency, Line Accuracy.

AI Debrief¶

LitertCoach (Gemma 4 E2B) generates a narrative session summary:

"Good session. Best lap 1:42.3, 3.1s behind AJ. Your Turn 3 improved — exit speed up 4mph from last session. Focus area for next session: Turn 7 entry. You're braking 20m too early and losing 0.8s per lap. Try the 3-board as your brake reference. Overall consistency improved — lap time spread down from 2.1s to 1.4s."

Mode Switching¶

The driver never picks a mode. The phone reads speed + GPS + a few timers and decides.

stateDiagram-v2
    [*] --> Boot: phone wakes
    Boot --> Idle: no telemetry
    Idle --> OnTrack: speed > 20 mph for 5 s
    OnTrack --> PitLane: speed < 30 mph AND in pit-lane geofence
    PitLane --> OnTrack: speed > 30 mph
    OnTrack --> CoolDown: speed < 5 mph for > 30 s
    CoolDown --> Paddock: speed < 5 mph for > 90 s
    Paddock --> OnTrack: speed > 20 mph
    Paddock --> Idle: 30 min idle
    Idle --> [*]: phone sleep

Five states, not two. The intermediate ones are the ones drivers hit on real track days.

State	When	UX
Boot	Just paired with hardware	Health check splash; "Bridge OK · Track loaded · Earbuds connected"
Idle	Sitting in paddock, engine off	Dashboard visible; coaching off
On-track	Active driving	Signal-light HUD; full coaching
Pit lane	Coming in or rolling out	HUD on; coaching reduced to safety only; no debrief generation (the lap isn't done)
Cool-down	Just stopped, < 90 s	HUD fades; quiet "Lap complete." chime + corner score chimes; not full paddock yet
Paddock	Stopped > 90 s	Engineer dashboard; on-device debrief begins generating in background

Edge cases that previously caused trouble¶

Scenario	Naive behaviour	What we do
Driver stops at corner-station for marshall	Falsely enters Paddock, kills the session mid-lap	Pit-lane geofence + 90 s cool-down threshold defer mode-switch until clearly off-track
Bridge process dies mid-lap	Coaching dies silently	HUD goes amber; sonic_model in-app fallback fires; "AI offline — basic cues only" spoken once and never again
Earbuds disconnect	Audio dies, driver hears nothing	HUD bars compress to bottom of screen, exposing 3 single-icon cards (brake / throttle / corner) — visual fallback. Cards are large, low-saturation, glanceable
Network drops	Cloud LLM (Gemini Flash for debrief) unavailable	Debrief generation queued; banner says "Debrief queued — will generate when online". The on-device coach (Gemma 4 E2B via LiteRT-LM) is unaffected
Low battery (< 15%)	App keeps running, drains	HUD darkens to AMOLED black, drops to 30 fps; non-essential signals (gold-standard overlay, weather) disabled. "Battery saver — coaching active" surfaced once
Mid-session brief stop (red flag, fuel)	App switches to Paddock and starts debriefing the half-session	Cool-down phase persists indefinitely while speed < 5 mph but red-flag flag is on; rejoining track resumes On-track without restarting the session_id
End-of-day shutdown	App closes, latest session never gets debriefed	Paddock-mode 30-min idle timeout flushes a final debrief + emits a session summary push notification

What persists across mode switches¶

The session_id is immutable for a track day. Every state change writes more rows to the same DuckDB session — laps, frames, signals, capabilities, coaching notes. This is what makes GET /driver/<id>/evolution work: every session is bracketed cleanly even when the driver dipped in and out of pit lane four times.

Accessibility & Sensory Constraints¶

A track-day cockpit is hostile to standard mobile-app UX. Helmet, gloves, glare, vibration, sweat, ear plugs. The product has to work in that environment for everyone.

Hearing protection + noise¶

Most drivers wear earplugs under the helmet for ear protection. Pixel Earbuds go on top. The Earbuds' tight fit + active noise cancellation actually helps — engine + tire noise is filtered, TTS comes through clearly. Volume calibration:

Cabin noise	Earbud volume	TTS speech rate
Pit lane idle	60%	1.0×
Out-lap (low rpm)	70%	1.0×
Hot lap straights (180 km/h)	85%	1.05×
Hot lap full throttle (~210 km/h)	100%	1.1×

Volume + rate auto-tune by speed_ms from the wide telemetry table. The driver never touches volume mid-session.

Hearing-impaired / deaf driver mode¶

Audio-first becomes audio-impossible. The Signal Light HUD takes over with expanded glyphs — same green/red bars as default, plus three large overlay icons:

┌─────────────────────────┐
│ 🅑 BRAKE     1.5G       │
│ 🅣 THROTTLE  18%        │
│ 🅒 CORNER    T7 entry   │
│ ┌───┐         ┌───┐     │
│ │   │         │   │     │
│ │ G │         │ R │     │
│ └───┘         └───┘     │
└─────────────────────────┘

Set in driver profile (audio_disabled: true). All audio is suppressed, all coaching becomes glyphs. Cadence is throttled to one new glyph every 2 s to prevent attention thrash.

Gloves¶

The driver wears gloves. The phone is in a windshield mount; the only on-track interaction is not touching the phone. Off-track, the paddock dashboard accepts:

Voice ("show me Turn 7", "play the best lap clip") via on-device speech recognition
Large-tap zones (minimum 64 dp tap targets)
No gesture-required interactions (no swipe-to-reveal, no long-press menus)

Glare + low light¶

The on-track HUD uses AMOLED-black backgrounds with high-saturation green/red bars. The phone screen brightness is locked at 100% during On-track, autodims in Paddock. The dashboard also offers a night mode that swaps green/red for cyan/magenta — easier on dilated pupils during dusk testing.

Vibration¶

The Pixel mount sits on the windshield where vibration is high. Fonts are minimum 24 sp; touch targets respect 12 dp deadband to absorb hand tremor.

Failure Modes — Always-On UX¶

The system must keep producing useful output when components fail. Each failure has a specific UX response — defined per ADR-009.

Failure	Driver-visible signal	Coaching impact	Recovery
Bridge process dies	HUD bars go amber once, "AI offline — basic cues only" spoken once	Falls back to in-app sonic_model rules; no LLM coaching, no debrief queue until recovery	Auto-reconnect attempted every 30 s
Cloud Gemini timeout	Banner in paddock dashboard: "Debrief queued — generates when online"	On-device coach (Gemma 4 E2B via LiteRT-LM) handles real-time coaching unaffected	Queued debrief generated on next network
Earbuds disconnect	Audio dies, three large glyph cards appear (brake/throttle/corner)	Coaching becomes visual; cadence drops to one cue per 2 s	Auto-pair on reconnect
Bluetooth telemetry drop (Racelogic / USB-CAN)	HUD bars freeze and dim 50%; "Telemetry lost" spoken once	Coaching uses last-known frame for ≤ 2 s, then suppressed	Auto-reconnect; resume mid-session_id
Network drop (cell + WiFi)	Tiny "○" indicator at corner of HUD; debrief panel shows queued state	On-device pipeline unaffected	Sync resumes when network returns
Low battery (< 15%)	"Battery saver" spoken once; HUD darkens to AMOLED black, drops 60→30 fps; gold-standard overlay disabled	Coaching continues at full quality	Plug in
Phone overheats	Frame rate drops to 15 fps, optional non-essential signals (weather, danger zones) disabled; "Cooling" spoken once	Coaching continues; on-device LLM may slow	Cool the phone (shade, fan)
GPS lost (tunnel, garage)	Bars don't move; "GPS lost" spoken once	Coaching pauses (no corner context)	Auto-resume on GPS lock
Track JSON missing	Boot fails — only state where the driver sees an error dialog (in paddock, not on track)	n/a — system won't proceed without a track	User picks a track from list

The pattern: one announcement, then go quiet. Repeated error speech mid-corner is worse than the failure itself.

Hardware Setup¶

The hardware story is intentionally minimal. Three things plug in, nothing else.

flowchart LR
  classDef car fill:#5d4a1a,stroke:#8a6e3a,color:#e0e0e0
  classDef phone fill:#1a3a52,stroke:#4a6e8a,color:#e0e0e0
  classDef cloud fill:#2e5d3a,stroke:#5a8a6e,color:#e0e0e0

  RL[Racelogic VBO Mini<br/>10 Hz GPS+IMU<br/>BT serial]:::car
  OBD[USB-CAN Adapter<br/>OBD-II via USB<br/>10 Hz]:::car
  PHONE[Pixel 10<br/>windshield mount<br/>USB-C charge]:::phone
  EAR[Pixel Earbuds<br/>BT audio]:::phone
  CLOUD[LitertCoach<br/>on-device — paddock debrief]:::cloud

  RL -->|BT serial| PHONE
  OBD -->|USB| PHONE
  PHONE -->|BT audio| EAR
  PHONE -->|local| CLOUD

What goes where¶

Item	Mounting	Power	Why
Racelogic VBO Mini	suction-cup on the dash, antenna with clear sky	car 12 V via cigarette lighter	The canonical 10 Hz GPS + IMU source. Per ADR-006, GPS clock anchors all telemetry.
USB-CAN Adapter	OBD-II port (driver footwell) → USB to Pixel 10	OBD-II port 12V	Brake / throttle / steering / RPM — the channels GPS doesn't have
Pixel 10	windshield mount, eye-level peripheral, charger run to centre console	USB-C from car 12 V → 65 W PD adapter	Compute + display + audio gateway. Stays charged for 4-hour track day.
Pixel Earbuds	in driver's ears, over foam earplugs	self-contained	Audio output. ANC + tight fit handles cabin noise.

What's not on the list: a smartwatch, a GoPro feed, a separate cellular hotspot, a cloud account. The only cloud dependency is the post-session debrief — and even that degrades gracefully (queued, not blocking).

First-run onboarding (driver setup)¶

The first time the app launches, a 90-second flow:

flowchart TD
  W1[Welcome — single screen<br/>Drive, get coaching, review.]
  W2[Pick driver name<br/>and skill level<br/>beginner / intermediate / pro]
  W3[Pick coaching persona<br/>T-Rod / Bentley / Drill Sergeant / Calm Pro / Buddy<br/>preview each with one sample line]
  W4[Pick car<br/>or skip — defaults to BMW M3 E46]
  W5[Pair Pixel Earbuds<br/>system dialog]
  W6[Pair Racelogic + USB-CAN<br/>auto-discover, plug USB, no pairing]
  W7[Pick today's track<br/>defaults to Sonoma]
  W8[Calibration: 10 s of telemetry<br/>verifies all sensors agree]
  DONE[Ready — go drive]

  W1 --> W2 --> W3 --> W4 --> W5 --> W6 --> W7 --> W8 --> DONE

The driver hits seven screens before the system is ready. After that, every subsequent session is zero-touch: power-on the car, the phone auto-pairs the same hardware, the same persona/level/track are used. The driver pulls onto the track and the system is already coaching.

Per-session pre-flight check¶

Even on the second session of the day, the system runs a 5-second health check before the driver pulls out:

Bridge      ✓
DuckDB      ✓ session-20260423-1004 ready
Track       ✓ Sonoma Raceway
Racelogic   ✓ 10.0 Hz, 11 sats
USB-CAN     ✓ 10.0 Hz
Earbuds     ✓ 87% battery
Coach       ✓ T-Rod / intermediate
Goals       ◐ 2 set (carry T7 apex, break 1:48)
            tap to add a third or skip

This is the only paddock screen the driver looks at before driving. Any failure here gets a one-tap "retry" before the car moves; once the car moves, the screen flips to On-track HUD and that's it.

Why no smartwatch / no haptic¶

Tested early; rejected. Wrist haptics during active driving are missed (the wrist is busy steering). Watch face is occluded by gloves. Adding a wearable adds another pairing step + battery to manage. The phone is enough.

Trust UX¶

The product name is trustable AI racing coach. Trust isn't a tagline — it's a UX surface. Three concrete mechanisms encode it.

1. Confidence shapes phrasing, not a number¶

A driver at 130 mph cannot read "AI confidence: 0.62". So confidence becomes part of the spoken cue, not a UI overlay. Per ADR-001, every coaching message carries a confidence score; the coach engine maps confidence to register:

Confidence	Phrasing	Volume	Example
high (>0.85)	Direct, imperative	100%	"Brake at the bridge."
medium (0.6–0.85)	Hedged, suggestive	90%	"You can probably brake later — try the 4-board."
low (0.4–0.6)	Question, observational	80%	"You're braking earlier than yesterday — was that on purpose?"
very low (<0.4)	Suppressed	—	(nothing said)

Drivers learn the register quickly. By session three, they know "you can probably" means AI is not certain, and they weight it accordingly. This is the trust surface — the AI never lies about how sure it is.

2. Provenance is one tap away¶

Every coaching phrase the AI ever speaks has a source. In the paddock dashboard, every line of the AI debrief is tappable — tapping reveals the provenance trail:

"Your Turn 3 improved — exit speed up 4mph from last session."
                              ↓ tap
                              ┌──────────────────────────────────┐
                              │ Source: telemetry burst #247     │
                              │ Frames 2812–2877                 │
                              │ Exit speed: 71 mph (this session)│
                              │            67 mph (last session) │
                              │ Compared to: AJ gold standard    │
                              │              68 mph              │
                              │ [Show on speed trace ▶]          │
                              └──────────────────────────────────┘

For coaching grounded in pedagogy, the citation reveals the source page:

"Trail-brake to the apex, smooth release."
                              ↓ tap
                              ┌──────────────────────────────────┐
                              │ From: Bentley, Performance       │
                              │       Driving Illustrated, p.84  │
                              │ Concept: trail_brake             │
                              │ T-Rod also: "Roll the brake to   │
                              │              the apex" (TROD_VOICE)│
                              └──────────────────────────────────┘

This is zero-effort provenance for the curious driver, invisible for the driver who just wants to drive. The information lives in the existing pipeline — bentley_concept is already attached to every CoachingMessage; we just need to render it tappable.

3. Disabled coaches explain themselves¶

When a coach rule can't fire because the session is missing a signal (per ADR-015 capability gating), the dashboard shows it explicitly:

Coaches active in this session
✓ Base pace note
✓ Trail-brake score
✓ Oil temp warning at T11

Coaches disabled
✗ Clutch balance — your car doesn't expose clutch position
✗ TPMS drift — pressure data is at 1.0 Hz, needs ≥5 Hz

The driver always knows what the coach isn't watching. No silent gaps, no "did the AI miss something or is it not tracking that?" anxiety. The capabilities envelope at GET /session/<sid>/capabilities is exactly this list.