What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
Minor road updates (like those in map data that might be a few months old if you're using maps from different regions) usually result in negligible cost differences for shortcuts, so the pre-calculated values remain effective.
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$600 $500 (17% off) Segway