Turnpike theory refers to a set of economic theories about the optimal path of accumulation (often capital accumulation) in a system, depending on the initial and final levels. In the context of a macroeconomic exogenous growth model, for example, it says that if an infinite optimal path is calculated, and an economic planner wishes to move an economy from one level of capital to another, as long as the planner has sufficient time, the most efficient path is to quickly move the level of capital stock to a level close to the infinite optimal path, and to allow capital to develop along that path until it is nearly the end of the desired term and the planner must move the capital stock to the desired final level. The name of the theory refers to the idea that a turnpike is the fastest route between two points which are far apart, even if it is not the most direct route.
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Origins
Although the idea can be traced back to John von Neumann in 1945, Lionel W. McKenzie traces the term to Robert Dorfman, Paul Samuelson, and Robert Solow's Linear Programming and Economics Analysis in 1958, referring to an American English word for a Highway:
Variations
McKenzie in 1976 published a review of the idea up to that point. He saw three general variations of turnpike theories.
Applications
The theorem has many applications in optimal control and in a general equilibrium context. In general equilibrium, the variation which involves infinite capital accumulation paths can be applied. In a system with many infinitely lived agents with the same (small) discount rates on the future, regardless of initial endowments, the equilibrium allocations of all agents converge.