tire-chain-law-navigator

Tire Chain Law Navigator

Overview

Winter mountain driving has a legal layer most drivers discover only at a checkpoint or in a ditch: chain law. California posts R1/R2/R3, Colorado alternates a Traction Law and Passenger Vehicle Chain Law, Washington and Oregon have their own codes, BC requires chains Nov 1–Mar 31 on many routes, and Alpine countries post snow-chain signs (267/268) that are legally binding. Drivers from out of region routinely get fined or turned around — or worse, drive unchained in conditions where chains were required.

This skill gives you:

1. Verdict by route + date + vehicle — which requirement class applies on a given pass/corridor in snow season, for your vehicle class (passenger / 4WD-AWD / CMV truck), with fine ranges.
2. Carry vs install rules — where chains must merely be carried (e.g. BC commercial vehicles, some park roads) vs installed, and the AWD-with-snow-tires exemptions (they exist — but not in R2 for 2WD, not at R3 for anyone).
3. Chain-up zone finder — how to locate the signed chain-up areas before a checkpoint (US: "Chain Up Ahead" signs; EU: panneau zones chaînes), and why you must never install on a live lane.
4. Sizing guide — cable vs link chains vs socks: what your clearance allows, what is legal where (socks are not legal substitutes in some jurisdictions/states for R2), speed limits with chains (typically 25–30 mph / 30–50 km/h).
5. Dash-card — a printable one-pager for the glovebox: your route's requirement classes, phone numbers (road conditions), checkpoint behavior.

The script holds a curated corridor database (I-70 Colorado, Donner Pass I-80, Snoqualmie I-90, BC Coquihalla, Alpine crossings) and computes seasonal requirements by date.

When to Use

• Planning a ski / mountain trip in snow season (Nov–Apr in most ranges)
• "Do I need chains for I-70 to Denver if it's snowing?" → verdict + exemptions
• Driving a rental or a new AWD vehicle into the mountains first time
• CMV/truck routing decisions in winter (chain requirements differ)
• Alpine driving (France/Italy/Switzerland/Austria): panel 267/268 meaning

Don't use for: real-time road status — always confirm with the live phone line/website listed on the dash-card the day of travel. This skill is the legal/rule layer, not a live feed.

Quick Start

# What applies on I-70 mountain corridor, Jan 15, passenger AWD with snow tires?
python3 scripts/chainlaw.py check --route i70-co --date 2027-01-15 \
    --vehicle awd-snows --conditions active

# Same corridor, 2WD all-seasons in active snow (what changes?)
python3 scripts/chainlaw.py check --route i70-co --date 2027-01-15 \
    --vehicle rwd-allseason --conditions active

# List every known corridor with its jurisdiction + season
python3 scripts/chainlaw.py routes

# Chain sizing / equipment legality primer
python3 scripts/chainlaw.py equipment --vehicle passenger --clearance tight

# Printable dash-card for the trip
python3 scripts/chainlaw.py dashcard --routes i70-co,donner-i80 --vehicle awd-snows --out card.md

Requirement Classes (core knowledge)

Key nuance the script encodes: AWD is not a chain exemption at R3 or CO's Passenger Vehicle Chain Law — and "snow tires" means 3PMSF or M+S with adequate tread, not all-seasons with highway tread.

How the Verdict Is Computed

1. Date → is it in the corridor's chain-law season? (Most: Nov 1–Mar 31; some passes Oct 1–May 31 — see routes output.)
2. Route → jurisdiction + default requirement class for that date (base = carry advisory; --conditions active escalates per the jurisdiction's escalation ladder).
3. Vehicle → exemptions applied (AWD+snows at R1/traction-law; none at R3).
4. Output: requirement class, whether YOU may pass, what to install, fine exposure, and the corridor's info line.

Common Pitfalls

1. "I have AWD, I don't need chains." True at R1/Traction Law with proper snow tires; FALSE at R2 in some states for 2WD (chains on drive axle), always FALSE at R3 and Colorado's PV Chain Law. AWD helps you go; it does not help you stop, and law is about control.
2. Socks vs chains. Auto socks are accepted in some jurisdictions as "traction devices" and not in others (CA accepts cables; some EU states accept textile; BC distinguishes). Check equipment output before relying on socks.
3. Speed with chains. 25–30 mph max — chains destroy tires/fenders and can wrap drivelines at speed. It's also the law in most jurisdictions.
4. Installing on the shoulder of a live lane. Use signed chain-up zones; they're flat, wide, and visible to trucks. People die doing this on live shoulders.
5. All-seasons ≠ snow tires. M+S marking with 4/32" tread may fail a checkpoint's tread check. CO traction law: ≥3/16" (≈5/32") tread.

Verification Checklist

• Route + date checked against the corridor season table
• Vehicle class + tire type entered honestly (tread depth matters)
• Live status confirmed via the corridor's info line on travel day
• Chains tried on the vehicle BEFORE the trip (fitting practice)
• Dash-card printed with phone numbers for every corridor on the route

One-Shot Recipes

Ski trip, Denver→Vail, AWD SUV with new snow tires:

python3 scripts/chainlaw.py check --route i70-co --date 2027-02-20 \
    --vehicle awd-snows --conditions active
python3 scripts/chainlaw.py dashcard --routes i70-co --vehicle awd-snows --out card.md

Rear-wheel-drive rental, Tahoe weekend:

python3 scripts/chainlaw.py check --route donner-i80 --date 2027-01-09 \
    --vehicle rwd-allseason --conditions severe
# → expect: at R2 you install chains on drive axle; carry them regardless.

Alps rental in January:

python3 scripts/chainlaw.py check --route alps-fr-it --date 2027-01-12 \
    --vehicle fwd-snows --conditions sign267