1. Road width isn’t fixed, not even on highly standardized highways, no way to determine metric depth.
2. Roads curve vertically so measuring distance by lines, even with a fair amount of parallax by motion window, will error (only bounded by the fact that road width bounded)
You'd probably have slightly better luck using the width of the lane stripes, those are painted mechanically.
But if I was serious about this, I'd like the size of stop signs, and the font size on highway signs, both of which seem like they'd be standardized locally.
The distance between the left side of a lane and the right side is approximately constant over distances you care about. And by the time you care about more accuracy you’ll be closer and can do easier things.
Oh interesting! I wonder if road sign sizes are used for this - they’re *mostly* standardized by regulation, but deviation from the standard is permitted for visibility/space, so not a great ground truth.
Unrelated but I once went on a traffic sign history rabbit hole and found the interesting fact that the US spearheaded the effort for the first/onld global standard in the UN, got approved by most countries... and then the US refused to ratify it themselves lol https://en.wikipedia.org/wiki/Vienna_Convention_on_Road_Signs_and_Signals
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2. Roads curve vertically so measuring distance by lines, even with a fair amount of parallax by motion window, will error (only bounded by the fact that road width bounded)
But if I was serious about this, I'd like the size of stop signs, and the font size on highway signs, both of which seem like they'd be standardized locally.