traffic2.gms : Traffic Assignment Model

Description

```In this traffic assignment problem, we have a number of users/cities, connected
by a shared network of roads.   Each user n
needs to move a fixed quantity from her city n to city n++3
over the shared network.  Each user can route flow along multiple paths between
her source and destination nodes.  The delay on each path is the sum of the
delay along each arc in the path, while the *effective delay* for each user
is the maximum delay over her paths with nonzero flow.  Each user
routes her flow so as to minimize her effective delay, assuming the flow chosen
by the other users is constant.

We give two formulations of the model: the first is as a VI via EMP,
the second as a standard MCP.  The MCP formulation has the advantage of yielding
a small model (one variable per city) but the function used to define the MCP
is less readable and maintainable than the functions used to define the VI.
It would be easier to modify the VI formulation to add some additional links
or to bound some of the link flows, while the MCP formulation depends on the
particular form of the network.

The MCP formulation is taken from

Steven P. Dirkse,  Ph.D. Dissertation
Robust Solution of Mixed Complementarity Problems.
Mathematical Programming Technical Report 94-12, August 1994.
ftp://ftp.cs.wisc.edu/math-prog/tech-reports/94-12.ps
Chapter 3.2.4, Pages 63-65

while the model originates from

Bertsekas, D. P. & Gafni E. M. (1982) 'Projection methods for variational
inequalities with application to the traffic assignment problem',
Mathematical Programming Study 17, 139-159

Contributor: Steven Dirkse, February 2009
```

Small Model of Type : VI

Category : GAMS EMP library

Main file : traffic2.gms

``````\$title Traffic Assignment Model (TRAFFIC,SEQ=19)

\$ontext

In this traffic assignment problem, we have a number of users/cities, connected
by a shared network of roads.   Each user n
needs to move a fixed quantity from her city n to city n++3
over the shared network.  Each user can route flow along multiple paths between
her source and destination nodes.  The delay on each path is the sum of the
delay along each arc in the path, while the *effective delay* for each user
is the maximum delay over her paths with nonzero flow.  Each user
routes her flow so as to minimize her effective delay, assuming the flow chosen
by the other users is constant.

We give two formulations of the model: the first is as a VI via EMP,
the second as a standard MCP.  The MCP formulation has the advantage of yielding
a small model (one variable per city) but the function used to define the MCP
is less readable and maintainable than the functions used to define the VI.
It would be easier to modify the VI formulation to add some additional links
or to bound some of the link flows, while the MCP formulation depends on the
particular form of the network.

The MCP formulation is taken from

Steven P. Dirkse,  Ph.D. Dissertation
Robust Solution of Mixed Complementarity Problems.
Mathematical Programming Technical Report 94-12, August 1994.
ftp://ftp.cs.wisc.edu/math-prog/tech-reports/94-12.ps
Chapter 3.2.4, Pages 63-65

while the model originates from

Bertsekas, D. P. & Gafni E. M. (1982) 'Projection methods for variational
inequalities with application to the traffic assignment problem',
Mathematical Programming Study 17, 139-159

Contributor: Steven Dirkse, February 2009

\$offtext

\$eolcom //

sets
n   'cities/origins in the network'     / n1 * n5 /
typ 'types of nodes' /
od      'origin-destination node'
oA, oB  'outer loop'
iA, iB  'inner loop'
/
p   'path types' / inner, outer /
;
alias (n,n1,n2);
alias (typ,t1,t2);
sets
A(n1,t1,n2,t2) 'unidirectional arcs from (n1,t1) to (n2,t2)'
paths(n,p,n1,t1,n2,t2) 'path-arc incidence matrix';

A(n1,'od',n1   ,'oA') = yes;   // on-ramp to outer loop
A(n1,'oA',n1   ,'oB') = yes;   // main outer arc
A(n1,'oB',n1++1,'od') = yes;   // off-ramp from outer loop
A(n1,'oB',n1++1,'oA') = yes;   // outer loop by-pass

A(n1,'od',n1   ,'iA') = yes;   // on-ramp to inner loop
A(n1,'iA',n1   ,'iB') = yes;   // main inner arc
A(n1,'iB',n1--1,'od') = yes;   // off-ramp from inner loop
A(n1,'iB',n1--1,'iA') = yes;   // outer loop by-pass

abort\$[card(A) <> 8*card(n)] 'We should have 8 arcs per city';

scalars
w       'highway weight'       / 10 /
gamma   'coupling coefficient' / 0.5 /;
* The delay on some links is increased by
*    gamma * delay on merging/exiting links
* with gamma set to  0, 0.5, or 4

\$ONTEXT
demands are either
( .1,   .2,     .3,     .4,     .5)
or
( 1,    8       1       8       1),
\$OFFTEXT

parameters
demand (n) /
n1   .1
n2   .2
n3   .3
n4   .4
n5   .5
/,
c(n,p,n1,t1,n2,t2)  'coefs for delay';

c(n,'outer',n,   'od',n,   'oA') = 1;     // main node to outer loop
c(n,'outer',n,   'oA',n,   'oB') = w;     // outer loop
c(n,'outer',n,   'oB',n++1,'oA') = 1;     // outer loop bypass
c(n,'outer',n++1,'oA',n++1,'oB') = w;     // outer loop
c(n,'outer',n++1,'oB',n++2,'oA') = 1;     // outer loop bypass
c(n,'outer',n++2,'oA',n++2,'oB') = w;     // outer loop
c(n,'outer',n++2,'oB',n++3,'od') = 1;     // outer loop to main node

c(n,'inner',n,   'od',n,   'iA') = 1;     // main node to inner loop
c(n,'inner',n,   'iA',n,   'iB') = w;     // inner loop
c(n,'inner',n,   'iB',n--1,'iA') = 1;     // inner loop bypass
c(n,'inner',n--1,'iA',n--1,'iB') = w;     // inner loop
c(n,'inner',n--1,'iB',n--2,'od') = 1;     // inner loop to main node

abort\$[card(c) <> 12*card(n)] 'We should have 12 arcs per city in c';

paths(n,p,n1,t1,n2,t2) = c(n,p,n1,t1,n2,t2);

* now add some extra delay for congestion effects due to arcs not on the path
* two types of congestion are considered:
*   1. delay on an on-ramp due to merging with traffic on a bypass link
*   2  delay on a highway due to traffic on an off-ramp
c(n,'outer',n--1,'oB',n   ,'oA') = gamma;    // bypass traffic slows on-ramp
c(n,'outer',n   ,'oB',n++1,'od') = 2*gamma;  // off-ramp traffic slows highway
c(n,'outer',n++1,'oB',n++2,'od') = 2*gamma;  // off-ramp traffic slows highway
c(n,'outer',n++2,'oB',n++3,'od') =
c(n,'outer',n++2,'oB',n++3,'od') +2*gamma;  // off-ramp traffic slows highway

c(n,'inner',n++1,'iB',n   ,'iA') = gamma;    // bypass traffic slows on-ramp
c(n,'inner',n   ,'iB',n--1,'od') = 2*gamma;  // off-ramp traffic slows highway
c(n,'inner',n--1,'iB',n--2,'od') =
c(n,'inner',n--1,'iB',n--2,'od') +2*gamma;  // off-ramp traffic slows highway

positive variables
x(n,p)         'flow on each path';
variables
f(n1,t1,n2,t2) 'flow on each arc';

equations
delay(n,p)     'delay on each of the paths'
fDef(n1,t1,n2,t2)
flowCon(n)    'flows must meet demands'
;
\$macro d(f) (1 + f + sqr(f))

delay(n,p)..  sum{A, c(n,p,A)*d(f(A))} =N= 0;
fDef(A)..  f(A) =e= sum{paths(n,p,A), x(n,p)};
flowCon(n).. sum{p, x(n,p)} =G= demand(n);

model mVI / delay, fDef, flowCon /;

* define the complementary pairs for vi
file myinfo / '%emp.info%' /;
putclose myinfo 'vi delay x';

x.l(n,p) = 0.5 * demand(n);
f.l(A) = sum{paths(n,p,A), x.l(n,p)}

solve mVI using EMP;
abort\$[mVI.modelstat <> %MODELSTAT.LOCALLY OPTIMAL%] 'bad modelstat for mVI';
abort\$[mVI.solvestat <> %SOLVESTAT.NORMAL COMPLETION%] 'bad solvestat for mVI';

* for the report labels, ed = effective delay
parameter report(n,*), report2(n,p,*);
report2(n,p,'delayVI') = sum{A, c(n,p,A)*d(f.l(A))};
report2(n,p,'flowVI')  = round(x.l(n,p),8);
report(n,'ed_calcVI') = smax{p\$[x.l(n,p) > 1e-8], report2(n,p,'delayVI')};
report(n,'ed_margVI') = flowCon.m(n);
report(n,'ed_diffVI') = abs(report(n,'ed_calcVI')-report(n,'ed_margVI'));
report(n,'out-in_VI') = (report2(n,'outer','delayVI')
- report2(n,'inner','delayVI')) + eps;
display report, report2;

\$ontext
We can write the VI above without using auxiliary vars f and d.
Using an additional reformulation trick, we can write it as a VI(F,z), where
z(n) is the flow from n to n++3 on the (longer) outside loop,
demand(n) - z(n) the flow on the (shorter) inside loop, and
F(z) = delay(outer loop) - delay(inner loop)
Since the feasible set for z is a box, this is just a straight-up MCP.
\$offtext

positive variable z(n);
z.up(n) = demand(n);
equation   dd(n)    'delay(outer) - delay(inner), shipping n to n++3' ;

dd(n) ..
* delay(outer)
[
(1 + z(n) + power(z(n),2))
+ gamma * (1 + z(n++3) + z(n++4) + power(z(n++3)+z(n++4),2))

+ w * (1 + z(n) + z(n++3) + z(n++4) + power(z(n) + z(n++3) + z(n++4),2))
+ 2 * gamma * (1 + z(n++3) + power(z(n++3),2))

+ (1 + z(n) + z(n++4) + power(z(n) + z(n++4),2))

+ w * (1 + z(n) + z(n++1) + z(n++4) + power(z(n) + z(n++1) + z(n++4), 2))
+ 2 * gamma * (1 + z(n++4) + power(z(n++4),2))

+ (1 + z(n) + z(n++1) + power(z(n)+z(n++1),2))

+ w * (1 + z(n) + z(n++1) + z(n++2) + power(z(n) + z(n++1) + z(n++2), 2))
+ 2 * gamma * (1 + z(n) + power(z(n),2))

+ (1 + z(n) + power(z(n),2))
]

=n=

* delay(inner)
[
(1 + demand(n)-z(n) + power(demand(n)-z(n),2))
+ gamma * (1 + demand(n++1)-z(n++1) + power(demand(n++1)-z(n++1),2))

+ w * (1 + demand(n)-z(n) + demand(n++1)-z(n++1)
+ power(demand(n)-z(n)+demand(n++1)-z(n++1),2))
+ 2 * gamma * (1 + demand(n++1)-z(n++1) + power(demand(n++1)-z(n++1),2))

+ (1 + demand(n)-z(n) + power(demand(n)-z(n),2))

+ w * (1 + demand(n)-z(n) + demand(n++4)-z(n++4)
+ power(demand(n)-z(n)+demand(n++4)-z(n++4),2))
+ 2 * gamma * (1 + demand(n)-z(n) + power(demand(n)-z(n),2))

+ (1 + demand(n)-z(n) + power(demand(n)-z(n),2))
];

model mMCP / dd.z /;
solve mMCP using mcp;
abort\$[mMCP.modelstat <> %MODELSTAT.OPTIMAL%] 'bad modelstat for mMCP';
abort\$[mMCP.solvestat <> %SOLVESTAT.NORMAL COMPLETION%] 'bad solvestat for mMCP';

* put together a report from the MCP solution
parameters xp(n,p), fp(n1,t1,n2,t2);
xp(n,'outer') = z.l(n);
xp(n,'inner') = demand(n) - xp(n,'outer');
fp(A) = sum{paths(n,p,A), xp(n,p)};

report2(n,p,'delayMCP') = sum{A, c(n,p,A)*(1 + fp(A) + sqr(fp(A)))};
report2(n,p,'flowMCP')  = xp(n,p);
report2(n,p,'flowDiff') = abs(xp(n,p) - report2(n,p,'flowVI')) + eps;
report(n,'ed_MCP')      = smax{p\$[xp(n,p) > 1e-8], report2(n,p,'delayMCP')};
report(n,'ed_diffMCP')  = abs(report(n,'ed_calcVI')-report(n,'ed_MCP')) + eps;
report(n,'out-in_MCP')  = dd.l(n) + eps;
report(n,'out-in_diff') = (report(n,'out-in_VI')-report(n,'out-in_MCP')) + eps;
display report, report2;

abort\$[smax{n, report(n,'ed_diffMCP')} > 1e-5] 'different effective delay in MCP and VI';
abort\$[smax{(n,p), report2(n,p,'flowDiff')} > 1e-5] 'different path flow in MCP and VI';
abort\$[smax{(n,p), report(n,'out-in_diff')} > 1e-5] 'different difference in MCP and VI';
``````