How we collect and check the data

The index of installation centers

We index 10,531 certified installation points across 4,901 cities and 51 states. They are compiled from the providers’ own public location directories — the same pages a customer would be sent to — and then reduced to unique physical places.

That reduction is the part worth explaining. The raw listings contain the same garage several times over, because two or three brands often operate out of one shop and each publishes it as its own location. We normalise the address and merge those into a single entry that carries every brand present at it. So a count on this site is a count of places you can drive to, not of listings — which is why our number for a city is often lower than the sum of the vendors’ own claims, and why a center can appear under two brands at once.

Coordinates come from the US Census Bureau’s public batch geocoder, which resolved 10,476 of them. Distances shown on city pages are computed from those coordinates to the city centre. Where an address did not resolve, the center is listed last and marked “distance not available” rather than given an invented figure.

State registries, where they exist

Some states publish their own list of approved service centers, and where they do we use it. 757 centers on this site carry data taken straight from a state registry: 199 of them a telephone number, the rest a certificate number and county.

One thing we deliberately do not take from those registries is the phone number a state lists as the “main number” when it turns out to be the manufacturer’s national line. In Utah’s list one such number appears against twenty-two of forty-three centers. Publishing it as the phone of a particular workshop would be exactly the thing we criticise vendors for, so those numbers are filtered out and the field stays empty.

Legal and price figures

Every claim about the law or the price is entered with the document behind it and the date it was checked. The registry currently holds 802 such claims drawn from 148 distinct sources, 118 of them government or court documents.

The check is tied to the wording, not to a calendar. Each claim stores a hash of the sentence that carries it; edit the sentence and the claim returns to the queue automatically. That is deliberate: a rewritten sentence can quietly change what a statute says, and a date alone would not catch it.

Prices are only published when the state itself publishes them — 14 jurisdictions do, 4 of them as a binding cap. Everywhere else we say the state publishes nothing and give the national reference from NHTSA, labelled as a reference rather than as your state’s price. We never republish a vendor’s “from” rate as though it were a price.

What we refuse to publish

• Ratings and star scores. We have no scoring method we could defend line by line, and marking up a rating that does not exist is structured data spam. Comparisons here rank on things that can be counted.
• Instructions for defeating the device. The search demand exists and we answer it with what actually happens next, citing the statutes, rather than with a method.
• Invented figures. Where a state publishes nothing, the field says so. An empty cell is more useful than a plausible average, because a reader will act on the average.
• Paid placement. No provider pays to be listed or ranked. If that ever changes it will be said here and on the funding page before it happens anywhere else.

Corrections

Laws change mid-year and fee schedules are reissued without announcement. If a figure here is out of date or wrong, tell us which page and which line and we will check it against the source and correct it — the check date on the page is there so you can see how old our reading is. How to reach us.

The rules this page describes are enforced by scripts that run before every release: a claim without a source blocks the build, and so does a price figure that renders as zero. Our editorial policy covers what we write about and why.