There are three common slot car scales (sizes): 1:24 scale, 1:32 scale, and so-called HO size. These are also commonly written as 1/24, 1/32, 1/87 and 1/64.
In 1957, Minimodels (UK) converted its Scalex 1:30 (later, 1:32) clockwork racers to electricity, creating the famous Scalextric line of slot-guided models, and Victory Industries (UK) introduced the VIP line, both companies eventually using the new plastic-molding technologies to provide controllable slot racers with authentic bodies in 1:32 scale for the mass market. Both lines included versatile sectional track for the home racer - or the home motorist; VIP produced sports cars and accessories slanted toward a "model roadways" theme, while Scalextric more successfully focused on Grand Prix racing.
As Scalextric became an instant hit, American hobbyists and manufacturers were adapting 1:24 car models to slots, and British-American engineer Derek Brand developed a tiny vibrator motor small enough to power model cars roughly in scale with HO and OO electric trains. In 1959, Playcraft division of Mettoy produced these in the UK, and a year later, Aurora Plastic Corp. released HO vibrator sets with huge success in the USA. The tiny cars fascinated the public, and their cost and space requirements were better suited to the average consumer than the larger scales. In only a year or two, Scalextric's 1:32 cars and Aurora's "Model Motoring" HO line had set off the "slot car craze" of the 1960s.