How Bridget detects drawbridge delays

Methodology & accuracy

The short version

Bridget has no backend and no live bridge feed. It infers, on your iPhone, when a drawbridge is backing up traffic — usually because it's raised — by comparing the live drive time across the bridge's approach to that road's normal free-flow time. A big, sustained slowdown is the signature of a stopped span.

It is honest about two limits: it reacts to congestion as it forms (so it helps you avoid a jam, it doesn't predict a raise), and it's only confident on the bridges where city records back it up.

How the signal works

For each bridge, Bridget derives a canonical probe — a point ~500 m back along the real approach road (validated to sit within 200 m of the bridge centerline, so it's on the actual approach, not a parallel street).
While you're on a planned route, it periodically asks Apple's MapKit for the live drive time from that probe to the bridge, and divides by the road's free-flow baseline. That ratio is the Congestion Index (CI).
A per-bridge, time-of-day calibrated threshold turns CI into a status. The calibration is done offline (see below) and baked into the app as fixed numbers — not fetched.

How we validated it

We collected CI every ~3 minutes for all seven Seattle bridges over a week (~23,000 readings) and cross-checked every reading against the City of Seattle's official drawbridge opening records (SDOT Open Data, dataset gm8h-9449) — 248 real openings in the window. We then measured, per bridge: how reliably CI catches a real opening (recall), how often it fires on normal traffic (false alarms), separability (ROC-AUC), and — combining our measurements with the city's long-term opening frequency — the calibrated probability that a flagged bridge is genuinely delayed.

What we found (per bridge)

Bridge	Confidence	Notes
Ballard	High	Catches ~97% of openings; calm baseline; the clearest signal. Alerts all day.
Montlake	Medium	Catches openings reliably, but its normal rush-hour traffic also spikes — so it alerts off-peak only, when a spike is meaningful.
Fremont	Low (board-only)	Opens most often, but short openings let traffic divert, so CI can't cleanly separate them. Shown on the board; no proactive alerts.
South Park	Promising, not yet validated	Excellent separation but too few openings in the window to confirm. Board-only for now.
1st Ave S	Board-only	Opens rarely; congestion there is usually port/rail, not the bridge.
University, Spokane St	Board-only	Absent from the city feed during the study (no openings recorded), so we have no ground truth to validate them yet.

Honest limitations

It detects backups, not raises. Congestion takes a few minutes to build, so CI typically crosses its threshold after a bridge has started opening. That's fine for avoiding a queue you haven't reached yet — but Bridget can't promise a warning before the span goes up.
Openings are rare, so even a good signal yields modest precision: when Bridget flags a bridge, the chance it's a confirmed opening (rather than ordinary heavy traffic) ranges from roughly 1-in-12 to nearly 1-in-2 depending on the bridge. We surface confidence per bridge rather than implying certainty.
It's an inference, not a sensor. Bridget reads traffic; it does not receive any official bridge-status signal.
Calibration is a snapshot. Thresholds are tuned to the records available at build time and refined as we collect more data.

What we never do

We never use a live city or government feed in the app, never send your data anywhere, and never describe Bridget as receiving official bridge status. The city records are used only offline, to tune the thresholds — the shipping app has no servers.