Let's be cautious attributing things to 'efficiency improvements' - that is a minefield of ways to be wrong [0]. Is it not a reasonable theory that the tailing off of demand is linked to increasing real prices of gasoline [1]? That seems like a simple and obvious explanation to me. I don't know if my source is particularly reliable, but real petrol prices have been all over the shop in recent years.

[0] https://en.wikipedia.org/wiki/Jevons_paradox

[1] https://inflationdata.com/articles/inflation-adjusted-prices...