The First World War had important repercussions for Glasgow. Many of the city's most able and energetic young men were killed, cutting out almost a whole generation of the entrepreneurs who might have ensured the city's continuing prosperity. But it also encouraged the development of large-scale manufacture of heavy motor vehicles and diffused the knowledge of how to maintain and operate them. The availability of cheap war-surplus chassis was a potent stimulus to the development of motor-bus and lorry services, both of which eroded the pivotal position of the railways to an increasing extent during the inter-war period.
Major new trunk motor roads were built in the 1920s and 1930s to Edinburgh, Ayr and to Inverness via Fort William. In the 1920s a good deal of unbridled competition took place between small bus companies but, after regulation was introduced in 1930, a few large companies developed substantial businesses. Within Glasgow the Corporation bus and tram services had a monopoly, but by 1939 there were well-developed medium and long-distance bus services operating from the city's three formal and one informal bus stations. In the 1930s the development of mass-produced steel-bodied motor cars brought motoring within the means of a broad spectrum of the population, encouraging the development of car-based suburbs.
Though the Second World War and its aftermath halted this trend, it was well under way again by the mid-1950s. This period also saw most of the city's electric tram services replaced, in the 1940s and 1950s, first by trolley buses and then by motor buses. From the 1920s motor lorries steadily ate into the railways' goods business making much of it uneconomic by the late 1950s. It was not until the 1960s, however, that motor transport seriously eroded the traditional concept of the urban settlement.