Massive rises in private car ownership, firstly with the average working class man, then his wife and young adult offspring.
In UK this happened around late 1960s to late 1970s for the first (also with a rise in married women getting their own cars around 1970s/1980s) and 1990s/00s for the second.
I collect old motoring books (both those aimed at the average driver and training textbooks for mechanics) from 20th/early 21st century and can track these changes from the design/typography and writing styles over the years..
@ChristianHachm @disorderlyf @infobeautiful
The rise in second/third car ownership in Western nations since the 1980s would also have heavily encouraged this (by increasing general consumption and demand worldwide, especially as more manufacturing work got offshored to the Global South countries)