With a post-WW2 POD, what if one or many Western European countries enacted Microcar regulations that roughly mirror Japan’s Kei Car class yet amongst other things can also be driven at minimum on a motorcycle license, with further incentives given to indigenous low-volume carmakers or even motorcycle manufacturers interested in branching out?
In the context of the UK for example, OTL UK Microcar regulations were typically limited to a lightweight 3-wheel configuration (without even a reverse gear in the early-1960s) and could only be driven on a motorcycle license in return for a lower rate of purchase tax, lower vehicle excise duty and lower cost insurance than comparable four-wheel cars, hence explaining the relative popularity of marques such as Reliant and Bond.
The ATL UK Microcar regulations would instead roughly correspond to the OTL 1976 Japanese Kei Car regulations in terms of initial maximum 550cc engine displacement (with 4-wheelers later limited to 700-750cc max by the early/mid-1970s whilst 3-wheelers are limited to 900cc max) with initially similar size limitations to the OTL 1955 Japanese Kei Car regulations along with a maximum weight limit of around 550kg.
Yet such ATL Microcars can still be driven on a motorcycle license at minimum in return for a lower rate of purchase tax, lower vehicle excise duty and lower cost insurance than comparable four-wheel cars in the class above (which includes the likes of the Mini, Fiat 600 and Hillman Imp that are each excluded from the Microcar class based on either size, weight and engine displacement).
For a frame of reference the most well-known OTL European car that comes closest to conforming to the ATL UK Microcar regulations had it actually been built in the UK is the Fiat 500, despite being 2cm wider under the 1955 Japanese Kei Car regulations.
https://en.wikipedia.org/wiki/Kei_car