Every earlier persistence-run lesson in this book fixed speed at 4 m/s and only ever scanned persistence time (chapter six's own persistence-time-source-scan). This scan holds persistence time at 3 s fixed and checks three speeds instead: 2 m/s gives run length 6 m, 4 m/s reproduces the book's own checked persistence speed exactly at 12 m, and 6 m/s gives run length 18 m. Speed is the other half of the deterministic run-length product, and it had never been scanned on its own.
Every earlier persistence-run lesson fixed speed at four metres per second and scanned persistence time; this scan holds persistence time fixed and scans speed itself. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Every earlier run-length row held speed fixed at four
Every prior persistence-run lesson in this book kept speed at 4 metres per second and only ever varied persistence time. Here persistence time stays 3 seconds, but speed itself drops to 2 metres per second, giving run length 6 metres.
2⋅3=6
Speed itself is a source factor, not just persistence time
Persistence time never changes across these rows. Speed alone rescales the checked run-length value, the same way persistence time did in this book's own persistence-time source scan.
4⋅3=12
A faster speed extends the run at the same persistence time
At speed 6 metres per second, the same persistence time gives run length 18 metres. Three rows separate speed from the persistence-time factor already scanned elsewhere in this book.