17t: MTC MNL Mode Choice Using DT¶

For this example, we’re going to re-create model 17 from the Self Instructing Manual, but using the DT data format.

Unlike for the DB based model, we won’t need to manipulate the data in advance of creating our model, because combinations of idca Format and idco Format variables can be done on-the-fly using broadcasting techniques for numpy arrays.

To build that model, we are going to have to create some variables that we don’t already have: cost divided by income, and out of vehicle travel time divided by distance. The tricky part is that cost and time are idca Format variables, and income and distance are idco Format variables, in a different table. Fortunately, we can use SQL to pull the data from one table to the other, but first we’ll set ourselves up to do so efficiently.

d = larch.DT.Example('MTC')


We don’t need to do anything more that open the example DT file and we are ready to build our model.

m = larch.Model(d)

from larch.roles import P, X

m.utility.ca = (
+ X("totcost/hhinc") * P("costbyincome")
+ X("tottime * (altnum <= 4)") * P("motorized_time")
+ X("tottime * (altnum >= 5)") * P("nonmotorized_time")
+ X("ovtt/dist * (altnum <= 4)") * P("motorized_ovtbydist")
)


The “totcost/hhinc” data is computed once as a new variable when loading the model data. The same applies for tottime filtered by motorized modes (we harness the convenient fact that all the motorized modes have identifying numbers 4 or less), and “ovtt/dist”.

for a in [4,5,6]:
m.utility[a] += X("hhinc") * P("hhinc#{}".format(a))


Since the model we want to create groups together DA, SR2 and SR3+ jointly as reference alternatives with respect to income, we can simply omit all of these alternatives from the block that applies to hhinc.

For vehicles per worker, the preferred model include a joint parameter on SR2 and SR3+, but not including DA and not fixed at zero. Here we might use a shadow_parameter (also called an alias in some places), which allows us to specify one or more parameters that are simply a fixed proportion of another parameter. For example, we can say that vehbywrk_SR2 will be equal to vehbywrk_SR.

m.shadow_parameter.vehbywrk_SR2 = m.parameter.vehbywrk_SR


Note that since the name of the second parameter contains a special character, we can’t just use the dotted version that we used for the first one (Python would complain about invalid syntax).

Having defined these parameter aliases, we can then loop over all alternatives (skipping DA in the index-zero position) to add vehicles per worker, etc. to the utility function:

for a,name in m.df.alternatives()[1:]:
m.utility[a] += (
+ X("vehbywrk") * P("vehbywrk_"+name)
+ X("wkccbd+wknccbd") * P("wkcbd_"+name)
+ X("wkempden") * P("wkempden_"+name)
+ P("ASC_"+name)
)


We want to calculate the standard errors for the model too:

m.option.calc_std_errors = True


We didn’t explicitly define our parameters first, which is fine; Larch will find them in the utility functions (or elsewhere in more complex models). But they may be found in a weird order that is hard to read in reports. We can define an ordering scheme by assigning to the parameter_groups attribute, like this:

m.parameter_groups = (
"costbyincome",
".*time",
".*dist",
"hhinc.*",
"vehbywrk.*",
"wkcbd.*",
"wkempden.*",
"ASC.*",
)


Each item in parameter_groups is a regular expression, which will be compared against all the parameter names. Any names that match will be pulled out and put into the reporting order sequentially. Thus if a parameter name would match more than one regex, it will appear in the ordering only for the first match.

Having created this model, we can then estimate it:

>>> result = m.maximize_loglike()
>>> result.message
'Optimization terminated successfully...

>>> m.loglike()
-3444.1...

>>> print(m)
====================================================================================================
Model Parameter Estimates
----------------------------------------------------------------------------------------------------
Parameter               InitValue       FinalValue      StdError        t-Stat          NullValue
costbyincome             0              -0.0524213       0.0104042      -5.03849         0
motorized_time           0              -0.0201867       0.00381463     -5.2919          0
nonmotorized_time        0              -0.045446        0.00576857     -7.87821         0
motorized_ovtbydist      0              -0.132869        0.0196429      -6.76423         0
hhinc#4                  0              -0.00532375      0.00197713     -2.69266         0
hhinc#5                  0              -0.00864285      0.00515439     -1.67679         0
hhinc#6                  0              -0.00599738      0.00314859     -1.90478         0
vehbywrk_SR              0              -0.316638        0.0666331      -4.75196         0
vehbywrk_Tran            0              -0.946257        0.118293       -7.99925         0
vehbywrk_Bike            0              -0.702149        0.258287       -2.71849         0
vehbywrk_Walk            0              -0.72183         0.169392       -4.26131         0
wkcbd_SR2                0               0.259828        0.123353        2.10638         0
wkcbd_SR3+               0               1.06926         0.191275        5.59018         0
wkcbd_Tran               0               1.30883         0.165697        7.89889         0
wkcbd_Bike               0               0.489274        0.361098        1.35496         0
wkcbd_Walk               0               0.101732        0.252107        0.403529        0
wkempden_SR2             0               0.00157763      0.000390357     4.04152         0
wkempden_SR3+            0               0.00225683      0.000451972     4.9933          0
wkempden_Tran            0               0.00313243      0.00036073      8.68358         0
wkempden_Bike            0               0.00192791      0.00121547      1.58614         0
wkempden_Walk            0               0.00289023      0.000742102     3.89465         0
ASC_SR2                  0              -1.80782         0.106123       -17.035          0
ASC_SR3+                 0              -3.43374         0.151864       -22.6106         0
ASC_Tran                 0              -0.684817        0.247816       -2.7634          0
ASC_Bike                 0              -1.62885         0.427399       -3.81108         0
ASC_Walk                 0               0.0682096       0.348001        0.196004        0
====================================================================================================
Model Estimation Statistics
----------------------------------------------------------------------------------------------------
Log Likelihood at Convergence           -3444.19
Log Likelihood at Null Parameters       -7309.60
----------------------------------------------------------------------------------------------------
Rho Squared w.r.t. Null Parameters      0.529
====================================================================================================
...


Tip

m = larch.Model.Example('17t')