Mirco Cars appears in Europe after WWII but proved not to be practical enough for every day use and the small cars replaced them.
Everyone remembers the Volkswagen "Bug", but there were other very popular makes in the late 50's and early 60's. In California the most popular ones included Hillman, Vauxhall, Simca, Renault, Morris Minor, and Austin American. Used models could be bought for $25-$100.
Most wore out long before they should because Americans drove them 60mph or faster, when they were designed more for speeds of 45mph. They would run fast but they just would not hold up.
Along come the "Bug" and it really didn't have enough power to tear itself apart. The valve train was designed so it could not be over rev'd and therefor became the standard for small cars.

The Smart Car is almost a Micro car. While I am sure you have seen news on the car, I thought I would show to one in traffic on the Champs Elysees
in Paris that I took last spring.
Click on the picture to get a true perspective on its size. Almost all the Smart Cars in Paris (as most cars) were diesel.
I could see myself driving one,
but I seriously lust for one of these little diesel panel trucks.