Transcript What is the Model??? A Primer on Transportation Demand

What is the Model???
A Primer on Transportation
Demand Forecasting Models
Shawn Turner
Theo Petritsch
Keith Lovan
Lisa Aultman-Hall
Objectives
• Provide overview of motor vehicle travel
demand forecasting process
• Discuss options and alternatives for
including biking and walking into vehicle
forecasting models
Origin
• Transportation Research Board
Committee on Bicycle Transportation
• Subcommittee recommendation January
2004
What’s the Problem?
• The “model” forecasts “tons” of motorized traffic
and so engineers must build bigger roads
• The “model” rarely includes peds and bikes
• In fact, the “model” structure and assumptions
do not work for bikes and peds
• But decision makers like numbers, so we need
to use other techniques to quantify the activity in
non-motorized transportation
The Model
1. Road Network
2. Zones in Study Area
3. Attributes of Zones
Traffic Volume by Road Link (section)
Defining the Study Area
1. Define boundaries
2. Establish the network
3. Create the zones
Defining the Study Area
1. Define Boundaries
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Numerous states have statewide models
Most models are used at the regional or
metropolitan planning organization level
-study area may be larger than appropriate
-statewide may be appropriate for tourism
PROBLEM
Defining the Study Area
2. Establish the network
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main arterial roads are represented as a series of links
and nodes
links are defined by speed and capacity
turns are allowed at nodes
Defining the Study Area
2. Establish the network
- links are defined by speed and capacity
- turns are allowed at nodes
-only main roads and intersections are included
-even collector roads are excluded
-off-road facilities are not included
PROBLEM
Defining the Study Area
3. Create traffic analysis zones
-uniform land use
-bounded by major roads
-neighborhood size (a few blocks or
more)
Defining the Study Area
3. Create the zones
-State of CT model – 2000 zones (5500
square miles and 3.4 million people)
-Lexington-Fayette County Ky - *** zones
(population 250,000 and 293 square
miles)
Defining the Study Area
3. Create the zones
-all modeled trips begin in a zone and
are destined for a zone
PROBLEM
-zone size is so large that most bike and pedestrian
trips start and end in the same zone and do not
use the network being modeled
-the typical zone attributes of population and
employment are not necessarily enough to predict
levels of biking or walking
The Model
1. Road Network
2. Zones in Study Area
3. Attributes of Zones
Traffic Volume by Road Link (section)
What’s in the Black Box?
• The four-step model
– Originally developed in the 1950s and 1960s
for interstate highway planning
– Many advanced newer modeling techniques
have been developed by researchers
– For the most part agencies still use the “four
step” model
The Four Step Model
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4.
Trip Generation
Trip Distribution
Mode Choice
Trip or Route Assignment
The Four Step Model
• Trip Generation – use of linear regression
to predict the number of trips beginning
from and destined for each zone based on
population and employment
PROBLEM
-there are relatively few bicycle and pedestrian trips
but note this process considers total trips at this
point, non-motorized trips have not necessarily
been excluded
-trip generation can be conducted by trip purpose
(work or shopping or total daily trips are common)
but note that recreational and discretionary trips
are hard to include
The Four Step Model
• Trip Distribution
– use of gravity models to link trip origin zones
to trip destination zones
– a trip matrix is produced
The Four Step Model
• Trip Distribution
– Trip interchanges increase with decrease
distance between zones
– Trip interchanges increase with increased
zone “attractiveness” (square footage of retail
or population for example)
The Four Step Model
• Trip Distribution
-all trips are still lumped together
-but bike and ped trips are affected differently
by distance
PROBLEM
-many bike and ped trips are intra-zonal and this
distance is hard to represent
The Four Step Model
• Mode Choice
– utility choice models predict the probability
that a trip between a pair of origin and
destination zones will be made by each mode
– drive alone, carpool, transit are commonly
included
– some agencies do little mode choice as most
of the trips they are modeling occur by
automobile
The Four Step Model
• Mode Choice
-Many agencies exclude bikes and peds completely
-The small percentage of bike/ped trips makes it
difficult to calibrate models and include them
PROBLEM
-Typical survey methods result in under-reporting of
discretionary trips, short trips and bike/ped trips
The Four Step Model
• Traffic or Route Assignment
– for a trip between an origin zone and a
destination zone the sequence of links (roads)
in the network for routing are selected
-often based on just minimum distance or time
-some models do more than one route for each
OD pair
The Four Step Model
• Traffic or Route Assignment
-The routes used by bikes and walkers are not
in the network
-Trips starting and ending within one zone are
PROBLEM
not assigned
-Factors beyond time and distance are also
important to non-motorized routing
The Four Step Model
• Conclusions for Bike and Ped Modes
– The geographic scale could be wrong
– The network is wrong
– The input variables are too limited
– Trip purpose is usually either too specific or
too general
Options and Alternatives
– 2 options
1. Tweak existing model or “post process”
intermediate or final model output
2. Use sketch planning tools outside of formal
model
– Burden of proof should be on transportation
professional
– Remind modelers how results will be used:
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Relative demand for prioritization
Mode shift potential