A glare check is only worth something if the sun is where the app says it is. That part is arithmetic, and it can be checked. The rest is either modelled with its assumption shown, or stated as not modelled. Nothing is hidden behind a confident-sounding number.
The sun
Glare computes the sun's altitude and bearing with the NOAA solar position equations: the equation of time, solar declination, hour angle, and the atmospheric refraction correction in all four of NOAA's altitude ranges. Sunrise and sunset are the moments the sun's centre crosses −0.833° (the standard 34′ of refraction plus 16′ of solar radius), found by a coarse scan and a bisection to the second.
The test suite compares this against two independent astronomical libraries, ephem and astral, across dates and latitudes. Agreement is within 0.1° in position and 20 seconds in sunrise and sunset time. Thirty minutes after sunrise in Detroit today the sun is at 4.8° altitude, bearing 87.2°; a planetarium app will give you the same figures to the decimal.
The road
Routes come from OSRM over OpenStreetMap. Each segment gets its own heading and its own arrival time from the routing engine's travel times, without live traffic. If you hit congestion, the later segments happen later than scored; a ten-minute delay moves the sun roughly 2° higher and 2° round the compass. The departure slider exists for exactly that.
The scoring
A segment scores glare when two things are true: the sun is low enough to sit under the visor (the vehicle preset, see the visor guide), and it is within 35° of the road's heading, with full weight inside 15°. Both are assumptions about a typical car and a typical driver's eye line. They are shown as such in the app's accuracy badges.
Not modelled
- Terrain. A hill in front of you hides a sun that is geometrically above the horizon. Not modelled.
- Buildings and trees. The 3D drive shows real building footprints blocking the disc, and the route score counts that occlusion where buildings are on the map. Trees, billboards and overpasses are not.
- Clouds. Shown separately from Open-Meteo's forecast as a confidence note. Never blended into the geometry, because a forecast is a forecast.
- Your windshield. Tint, dirt and a wet road turn a mild sun into a bad one. The app knows the geometry, not your glass.
- Other vehicles. A truck ahead of you is the best visor there is. Not modelled.
When one of these matters for your drive, the honest answer is that Glare tells you when the sun will be low and in front of you, and you decide what is between you and it.