Downs-Thomson paradox
Encyclopedia
Downs-Thomson paradox also referred to as the Pigou–Knight–Downs paradox (after Arthur Cecil Pigou
and Frank Knight
), states that the equilibrium speed of car traffic on the road network is determined by the average door-to-door speed of equivalent journeys by (rail-based or otherwise segregated) public transport.
It follows that increasing road capacity can actually make overall congestion
on the road worse. This occurs when the shift from public transport
causes a disinvestment
in the mode such that the operator either reduces frequency of service or raises fares to cover costs. This shifts additional passengers into cars. Ultimately the system may be eliminated and congestion on the original (expanded) road is worse than before.
The general conclusion, if the paradox applies, is that expanding a road system as a remedy to congestion is not only ineffective, but often counterproductive. This is also known as Lewis-Mogridge Position
and was extensively documented by Martin Mogridge with the case-study of London on his book Travel in towns: jam yesterday, jam today and jam tomorrow
?
An article of 1968 from Dietrich Braess now at the Faculty of Mathematics in Ruhr University, already pointed out the existence this counter-intuitive occurrence on networks - the Braess' paradox
states that adding extra capacity to a network, when the moving entities selfishly choose their route, can in some cases reduce overall performance.
There is a recent interest in the study of this phenomenon since the same may happen in computer networks and not only in traffic networks. Increasing the size of the network is characterized by behaviors of users similar to that of travelers on transportation networks, who act independently and in a decentralized manner in choosing their optimal routes of travel between origins and their destinations.
This is an extension of the induced demand
theory and consistent with Downs
(1992) theory of "triple convergence". Downs (1992) formulated this theory to explain the difficulty of removing peak-hour congestion from highways. In response to a capacity addition three immediate effects occur. Drivers using alternative routes begin to use the expanded highway, those previously traveling at off-peak times (either immediately before or after the peak) shift to the peak (rescheduling behavior as defined previously), and public transport users shift to driving their vehicles.
Arthur Cecil Pigou
Arthur Cecil Pigou was an English economist. As a teacher and builder of the school of economics at the University of Cambridge he trained and influenced many Cambridge economists who went on to fill chairs of economics around the world...
and Frank Knight
Frank Knight
Frank Hyneman Knight was an American economist who spent most of his career at the University of Chicago, where he became one of the founders of the Chicago school. Nobel laureates James M. Buchanan, Milton Friedman and George Stigler were all students of Knight at Chicago. Knight supervised...
), states that the equilibrium speed of car traffic on the road network is determined by the average door-to-door speed of equivalent journeys by (rail-based or otherwise segregated) public transport.
It follows that increasing road capacity can actually make overall congestion
Traffic congestion
Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction...
on the road worse. This occurs when the shift from public transport
Public transport
Public transport is a shared passenger transportation service which is available for use by the general public, as distinct from modes such as taxicab, car pooling or hired buses which are not shared by strangers without private arrangement.Public transport modes include buses, trolleybuses, trams...
causes a disinvestment
Disinvestment
Disinvestment, sometimes referred to as divestment, refers to the use of a concerted economic boycott, with specific emphasis on liquidating stock, to pressure a government, industry, or company towards a change in policy, or in the case of governments, even regime change...
in the mode such that the operator either reduces frequency of service or raises fares to cover costs. This shifts additional passengers into cars. Ultimately the system may be eliminated and congestion on the original (expanded) road is worse than before.
The general conclusion, if the paradox applies, is that expanding a road system as a remedy to congestion is not only ineffective, but often counterproductive. This is also known as Lewis-Mogridge Position
Lewis-Mogridge Position
The Lewis–Mogridge Position, named after D. Lewis and M. J. H. Mogridge, was formulated in 1990. It captures the observation that the more roads are built, the more traffic there is to fill these roads. Speed gains from some new roads can disappear within months if not weeks...
and was extensively documented by Martin Mogridge with the case-study of London on his book Travel in towns: jam yesterday, jam today and jam tomorrow
Jam tomorrow
Jam tomorrow or jam to-morrow is an expression for a never-fulfilled promise. It originates from Lewis Carroll's 1871 book Through the Looking Glass and What Alice Found There...
?
An article of 1968 from Dietrich Braess now at the Faculty of Mathematics in Ruhr University, already pointed out the existence this counter-intuitive occurrence on networks - the Braess' paradox
Braess' paradox
Braess's paradox, credited to the mathematician Dietrich Braess, states that adding extra capacity to a network when the moving entities selfishly choose their route, can in some cases reduce overall performance...
states that adding extra capacity to a network, when the moving entities selfishly choose their route, can in some cases reduce overall performance.
There is a recent interest in the study of this phenomenon since the same may happen in computer networks and not only in traffic networks. Increasing the size of the network is characterized by behaviors of users similar to that of travelers on transportation networks, who act independently and in a decentralized manner in choosing their optimal routes of travel between origins and their destinations.
This is an extension of the induced demand
Induced demand
Induced demand, or latent demand, is the phenomenon that after supply increases, more of a good is consumed. This is entirely consistent with the economic theory of supply and demand; however, this idea has become important in the debate over the expansion of transportation systems, and is often...
theory and consistent with Downs
Anthony Downs
Anthony Downs is a scholar in public policy and public administration, and since 1977 is a Senior Fellow at the Brookings Institution in Washington D.C..-Education:...
(1992) theory of "triple convergence". Downs (1992) formulated this theory to explain the difficulty of removing peak-hour congestion from highways. In response to a capacity addition three immediate effects occur. Drivers using alternative routes begin to use the expanded highway, those previously traveling at off-peak times (either immediately before or after the peak) shift to the peak (rescheduling behavior as defined previously), and public transport users shift to driving their vehicles.