MileCheckHighways don't stop at state lines, and neither should a trip log.
MileCheck watches your GPS position against each state's boundary as you drive. The moment you cross from, say, Washington into Oregon, the app knows before you do, usually within a few seconds of the sign going by.
Your trip log splits where the state line actually is
If you were logging a trip, that crossing doesn't just get noted. The app closes out the segment you were on and opens a new one in the state you just entered, on the same highway, in the same direction. Your trip record ends up with two segments instead of one long, technically-wrong line between a mile marker in Washington and an unrelated mile marker in Oregon. A small banner tells you it happened: "Entered Oregon, new trip started."
The same logic runs on shared pavement, not just at borders
Interstate 90 and US-93 overlap for stretches in Montana. Take an exit mid-overlap, and the app has to decide which road you're actually still on, without flickering back and forth every time a GPS ping lands a few feet closer to one route's data than the other's. That decision holds steady for as long as you're on the shared stretch, and only changes once you're clearly and consistently on one road alone.
A reading from the wrong state doesn't count
None of this is guesswork layered on top of a single location. It's built on the same principle as the mile marker itself: every reading the app trusts carries a record of which state's data it came from, and a reading from the wrong state never extends a trip logged in another one. A GPS fix a few seconds after a border crossing can still pick up an old signal from the state you just left. The app checks for that, not just the fastest read.
The result is a trip log that looks like the drive actually looked: one continuous highway, split correctly where the state line, or the road, actually changed.
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Related: How MileCheck finds your mile marker offline · How one app reads 50 different state DOT systems · All 50 states