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John Glen Wardrop was an English transport analyst
who developed Wardrop's first and second
principles of equilibrium.  

The concepts are related to the idea of Nash
equilibrium in game theory developed separately.
However in transportation networks, there are many
players, making the analysis more difficult than
in games with small numbers of players.

Network equilibrium models are commonly used for
the prediction of traffic patterns in
transportation networks that are subject to
congestion. The idea of traffic equilibrium
originated as early as 1924, with Frank Knight.

In 1952, Wardrop stated two principles that
formalize this notion of equilibrium and
introduced the alternative behavior postulate of
the minimization of the total travel costs.

Wardrop's first principle of route choice, which
is identical to the notion postulated by Knight,
became accepted as a sound and simple behavioral
principle to describe the spreading of trips over
alternate routes due to congested conditions. 

Wardrop's first principle states: The journey
times in all routes actually used are equal and
less than those which would be experienced by a
single vehicle on any unused route. Each user
non-cooperatively seeks to minimize his cost of
transportation. The traffic flows that satisfy
this principle are usually referred to as "user
equilibrium" (UE) flows, since each user chooses
the route that is the best. Specifically, a
user-optimized equilibrium is reached when no user
may lower his transportation cost through
unilateral action.

A variant on this is the stochastic user
equilibrium (SUE) wherein no driver can
unilaterally change routes to improve his/her
perceived travel times.

Wardrop's second principle states: At equilibrium
the average journey time is minimum. This implies
that each user behaves cooperatively in choosing
his own route to ensure the most efficient use of
the whole system.  Traffic flows satisfying
Wardrop's second principle are generally deemed
"system optimal" (SO).  Economists argue this can
be achieved with marginal cost road pricing.

The first mathematical model of network
equilibrium was formulated by Beckmann, McGuire
and Winsten in 1956.

== Reference ==
Wardrop, J. G., 1952.  Some theoretical aspects
of road traffic research, Proceedings,
Institution of Civil Engineers, PART II,
Vol.1, pp. 325-378.