I must admit that I once transported lumber in much the same way.
It was in 1969 about 25 kilometres north-west of Kirkland Lake, Ontario. We placed about 20 up to 18 foot long rough sawn spruce 2x6s across the back seat of my friend George’s 4 door Chevy and drove that way for about 10 kilometres. I drove my VW Beatle in the middle of the road in front of George in order to warn any oncoming traffic. It was a 2 lane gravel road not much wider than the load. We only encountered two vehicles (one from in front, one from behind) and completed the journey without mishap.