All-four is an urban public transport scheme first enunciated by the Brooklyn-Manhattan Transit Corporation (BMT—New York City) in the 1930s in which different transportation technologies are chosen and implemented in an integrated system.
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The four modes of transport included in the scheme are:
All of these are electrically driven from a remote power source, except the last, driven by on-board petrol engines, with the power transmitted to the driving wheels mechanically or by electric generation.
The concept was intended to cast each of the four modes as complementary, rather than competitive, in a unified transit system. Prior to promotion of this concept, the modes were often seen as separate entities, with interested parties promoting one or the other as "best," a competition that still exists today. With petrol buses especially, routes were sometimes set up in competition with street railway companies by often-illegal entrepreneurs, looking to skim the franchised companies' fares and profits. These vehicles, sometimes no more than an automobile following streetcar routes, were called jitneys, a reference to the nickel (US five cents) fare charged – see dollar van and share taxis in the US.
Running from the bottom of the list (petrol buses) to the top (rapid transit) all-four represents increasing capital-construction and infrastructure-maintenance costs balanced by increasing system capacity, speed of operation, operating efficiency and vehicle durability in terms of maintenance and useful life. These latter advantages result in lower capital costs going forward through longer amortization periods, lower operating costs due to efficiencies of electric and rail operation, and significantly lower labor costs due to larger vehicles and the ability to link rail vehicles into trains consisting of as many as eleven cars. Customer satisfaction also tends to increase as transport modes move up the hierarchy due to improved speed and ride comfort.
The BMT's concept was that each mode had a niche to fill, depending upon market demand. Petrol buses were a "starter" system, to provide service to areas of light traffic and new subdivisions. When and if demand outpaced the buses' economic usage, a new trolleybus or tram line would substitute for all or part of the petrol buses' route. If the choice was to be a trolleybus, most of that route's electrical infrastructure could be used for an upgrading to a full tramway. The heaviest tram routes in turn could plot the route of heavy rapid transit lines.
The BMT pioneered introducing petrol buses on a large number of marginal routes through its subsidiary Brooklyn Bus Corporation, and substituted a single trolleybus for a streetcar line, the Avenue C/Cortelyou Road line in Brooklyn. As the system was taken over by the City of New York in 1940, the BMT management never had the opportunity to bring the concept to full fruition.
The scheme is in use today, though not widely practiced. This was originally caused by the contraction of trolleybus and tram routes in North America and to a lesser extent in the rest of the world, but more recently by the continued contraction of market for trolleybuses, even as the market for tramways in the form of light rail and pre-Metros, conventional rapid transit, and even petrol buses in the form of bus rapid transit have gained.
Still, a few impressive systems exist. San Francisco, California, especially, has seen some of the best implementation of the hierarchy, with trolleybuses replacing diesel on some routes, and expansion of light rail lines and the BART rapid transit system. San Francisco has, upon occasion, been referred to as possessing an all-five system, if one accepts the San Francisco cable car system as having transit value beyond tourism. In Boston, Massachusetts, while all four types do exist to this day, the trolleybus was never used in the all-four sense, and instead was seen as a step towards downgrading what were once streetcar lines to cheaper bus lines. All-four also exists in Philadelphia, Pennsylvania, and Mexico City as well as a number of cities outside the Americas, including Moscow and Saint Petersburg in Russia and Athens in Greece.