Car Seat Stage Pilot
Overview
Car-seat stages (rear-facing → forward-facing harness → booster → adult belt) are governed by THREE clocks at once: the child's age, weight, and height — plus the specific seat's limits. Parents get this wrong constantly: turning the seat at age 1 because "their legs look cramped", boosting a 3-year-old because "she's tall for her age", or dropping the booster at 6 because "the belt looks fine". Mis-staging is one of the most common — and most dangerous — usage errors found in car-seat check events.
This skill converts a birthday (or age in months) plus optional measurements into a definitive stage verdict: what position, what seat type, what harness rule applies right now, which limits are close, and when the next transition is projected.
When to Use
- "My son just turned 2 — can I turn his seat around now?"
- "She's 4 and 42 lb — is it booster time?"
- "When can my kid stop using a booster?"
- "The seat manual says RF to 40 lb — my toddler is 38 lb, how long do we have?"
- "New baby arriving — what seat stage do we buy?"
- Any request involving child age + car seat position/type/transition.
Don't use for: installation help for a specific vehicle (angle, tightness, anchor locations) — that's a certified Child Passenger Safety Technician job; direct the user to a local check station.
How It Works — Numbered Steps
- Collect inputs: birthday (best) or age in months; weight in lb; height in inches; current seat type if known (typical vs extended-rear-facing convertible).
- Run the stage engine:
python3 scripts/car_seat_pilot.py --birthday 2024-03-15 --weight-lb 24 --height-in 31Add--jsonfor machine output,--belt-fitto force the 5-step belt-fit checklist. - Read the verdict block: POSITION NOW / SEAT NOW / HARNESS NOW.
- Check
WARNING:lines — they fire when a measurement is at/over a seat limit (transition now). - Use NEXT TRANSITION to answer "when": earliest of age-, weight-, and height-driven dates from a crude growth model (see references/growth-model.md).
- For booster-age children, run through the 5-step belt-fit test verbatim — all five must pass.
The Four Stages (quick reference)
| Stage | Position | Age guide | Size gate | Key rule |
|---|---|---|---|---|
| 1. Infant/convertible | Rear-facing | birth–2+ y | RF to seat max (often 40–50 lb) | Max out the seat, don't race the birthday |
| 2. Harness | Forward-facing | ~2–4+ y | ≥40 lb typical floor | Straps AT/ABOVE shoulders + top tether |
| 3. Booster | Forward-facing | ~4–10 y | until 4'9" + belt-fit pass | Lap+shoulder belt only, never lap-only |
| 4. Adult belt | Back seat | 8–12 y | 5-step test passes | Back seat until age 13 |
Common Pitfalls
- Treating age 2 as the goal for turning. Age 2 is the FLOOR, not the target — keep rear-facing until the seat's RF limits are exhausted (many seats go to 40–50 lb).
- Strap height direction. Rear-facing: at or BELOW shoulders. Forward-facing: at or ABOVE. This is the #1 harness error and flips at the transition.
- Forgetting the top tether. Forward-facing installs need the tether even with lower anchors.
- Boostering on height alone. A tall 3-year-old stays in the harness; maturity (sitting the whole ride) gates the booster as much as size does.
- Mixing belt + lower anchors unless the seat manual explicitly permits both.
- Expiring seats. Seats expire 6–10 years from manufacture (plastic fatigue) — check the label.
- After any crash most manufacturers (and NHTSA) say replace the seat, even if it looks fine.
Example
Parent: "Daughter is 30 months, 24 lb, 31 in — grandma says she can face forward now."
$ python3 scripts/car_seat_pilot.py --birthday 2024-03-15 --weight-lb 24 --height-in 31
POSITION NOW: FORWARD-FACING (age floor passed at 24 mo)
SEAT NOW: convertible or combination seat installed FF
HARNESS NOW: 5-point harness + top tether
NEXT TRANSITION: Booster window opens (age ~4 + 40 lb) — 2028-03-15
Answer: yes, she may face forward (age floor passed), but she is ONLY 24 lb —
the harness stage continues for years; booster talk is ~2 years early.
Safety Note
Guidance basis: AAP 2018 policy statement, NHTSA, Safe Kids Worldwide. The child's seat manual and state law always supersede this tool. When in doubt, direct to a free certified car-seat inspection station (NHTSA's "car seat inspection station" locator).
References
references/stage-rules.md— full rule set per stage, strap-position tables, tether rules, state-law variability, expiry and post-crash rules.references/growth-model.md— the crude lb/month and in/month growth curves used for transition-date projection, and their error bars.
