The document discusses key issues and limitations related to transportation modeling for planning purposes. It notes that models provide insights, not definitive answers, and results must be interpreted based on inherent uncertainties and limitations. Precision of model outputs must be balanced with accuracy in context of the planning question being addressed. The scale of the model and associated data may not match the scale of the specific planning issue.

1. MODELING FOR PLANNERS
PEEKING INSIDE THE BLACK BOX

2. THANK YOU! QUESTIONS?

3. How much does expected traffic on
Gold Rush Parkway drop when
we add the BRT facility?
A. I don’t know. The model is still running
B. 6,765.7 vehicle trips per day
C. Hmmm, actually, traffic increased!
D. 6% compared to the “No-Project” scenario

4. If we implement “Smart Growth”
land use policies, can we take Gold Rush
Parkway off our long-range project list?
A. But all I have is a traffic model!
B. The model says increased density leads to
increased tripmaking in all modes. Uh-oh!
C. Results suggest the parkway will still be
necessary unless the growth is accompanied
by robust transit at all times of day.

5. KEY ISSUES
CHOICE
MODELING
TRIP & TOUR
BASED MODELS

6. Inherent Risk Limitations of
and Uncertainty Modeling
Insights,
not
Answers
Precision Scale of Model
vs. vs.
Accuracy Scale of Question

7. Insights, not Answers:
How do we translate from “Results” to “Insights?”
Telling the story is harder than running the model.
All numbers are relative!
Results must be interpreted based on constraints and
limitations of the models.
The story includes the path to the results, not just
the final answers.

8. Inherent Risk and Uncertainty:
How do you think about uncertainty in model results?
How robust are your strategies, given certain
uncertainty?
Become One With Your Data.
Especially household and employment forecasts!
What makes these results liberal/conservative?

9. Precision vs. Accuracy
Highly Precise.
✦✦✦
✦
Highly Accurate. ✦✦

10. Precision vs. Accuracy
✦
Not as Precise. ✦
✦ ✦
And yet, Accurate. ✦ ✦
✦ ✦

11. Precision vs. Accuracy
Highly Precise….
But Not Accurate!
✦✦
✦✦
✦
✦
✦ ✦
(11,346.5 trips?)

12. Precision vs. Accuracy
✦
✦ ✦ ✦
✦ ✦
✦
Imprecise… ✦
…and inaccurate.
✦ ✦
(a.k.a. “Wrong”)

13. Precision vs. Accuracy
Your model is
✦
✦✦
✦ extremely precise.
✦✦
✦✦
✦✦
✦
✦✦
✦
It may or may not be
accurate, as well.
✦✦
✦ ✦✦ ✦✦ Your job is to
✦
✦✦ ✦ round liberally, and
✦ ✦ ✦✦ ✦ consider the context!

14. Limitations of Modeling
As modelers, we need to be honest in facing
the ways models are inappropriate
for the questions at hand.
As planners, you need to be aware of model and
data shortcomings, so you can make
informed decisions.