Trip Distribution
and Assignment
Modeling Methods
Traffic Impact Analysis
Modeling Methods
• Why
– Most manual distribution and
assignment techniques include
numerous subjective inputs
– Models offer an MPO-adopted tool to
aid in distributing and assigning traffic
Modeling Methods
• FSUTMS–Florida Standard Urban
Transportation Model Structure
– Florida’s standard model used in all 26
MPOs
– FSUTMS uses Cube Voyager software
engine
• Effective for site impact analysis
when combined with manual
techniques
– At a minimum, FSUTMS trip generation
outputs must be verified with manual
analysis (ITE)
Travel Demand Modeling
• Four-Step Process
Trip Generation
– Other auxiliary modules
include network building
and calculating zone-tozone travel times.
Trip Distribution
Mode Split
Trip Assignment
FSUTMS Trip Generation
SFDU MFDU Hotel
1 xxx xxx
xxx
2 xxx xxx
xxx
3 xxx xxx
xxx
4 xxx xxx
xxx
Employment
xxx xxx xxx
xxx xxx xxx
xxx xxx xxx
xxx xxx xxx
ZONEDATA
Generation
Special Generators
2 xxx xx xxx xx
3 xxx xx xxx xx
SPECGEN
TAZ
1
2
3
4
PRODS
xxxx
xxxx
xxxx
xxxx
ATTRS
xxxx
xxxx
xxxx
xxxx
Trip Generation
• Inputs:
– Number of homes and employees (ZONEDATA file)
in each traffic analysis zone (TAZ)
• Outputs:
– Number of Production trip ends (generated by
homes) and Attraction trip ends (generated by
employment sites) for each zone
Note: The model adjusts the number of A’s
(usually downward) until number of P’s and A’s in
the entire MPO are equal.
FSUTMS Trip Distribution
PRODS
Distribution
ATTRS
F R O M__
Person-Trip Table
1
2
3
4
1
xx
xx
xx
xx
T O______
2 3 4
xx xx xx
xx xx xx
xx xx xx
xx xx xx
Trip Distribution
• Inputs:
– P’s and A’s calculated in trip generation
• Outputs:
– The number of person-trips traveling between each
zone pair in the MPO area
• Production trip ends are matched up with
attraction trip ends from throughout the
network to form person-trips, using a gravity
model.
FSUTMS Mode Split
Mode Split
Vehicle-Trip Table
F R O M__
PersonTrip
Table
1
2
3
4
1
xx
xx
xx
xx
T O______
2 3 4
xx xx xx
xx xx xx
xx xx xx
xx xx xx
FSUTMS Mode Split
• Inputs:
– Person-trip table from trip distribution
• Outputs:
– Vehicle-trip table
• Vehicle occupancy factors or transit usage
equations are applied to convert persontrips to vehicle-trips
– Vehicle occupancies differ by trip purpose (e.g., work
trips generally have fewer passengers than
recreational trips).
FSUTMS Highway Assignment
VehicleTrip
Table
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Assignments
xxx
xxx
Network Link Volumes
From
To
Node Node
Volume
1001
1002
xxxx
1001
1003
xxxx
1002
1004
xxxx
Highway Assignment
• Inputs:
– Vehicle-trip table
• Outputs:
– Number of trips upon each roadway link
• Vehicles are routed along specific roadways
to arrive at the zone they desire to reach.
FSUTMS vs. ITE Trip Gen
Which is better for Site Impact Analysis?
• Intended Purpose
– FSUTMS
• Designed to replicate MPO areawide travel
patterns
• Calibration = Avg Link Volume-To-Count
comparisons
• NOT calibrated to zone-specific traffic
volumes
– ITE
• Designed to find correlation between land
use quantities and number of trips entering
and exiting specific types of developments
FSUTMS vs. ITE Trip Gen
Which is better for Site Impact Analysis?
• Application
– FSUTMS
• Does not calculate “Trips” for employment sites.
Rather, the model calculates “Attractions,” which
are only measures of relative attractiveness to
draw Productions to the zone.
– ITE
• Calculates the average number of trips generated
by varying quantities of specified land uses.
So what is FSUTMS good for?
• Key strengths:
– Trip distribution and assignment
– Non-development trips
• Therefore…
– The best modeling methodology for traffic
impact analysis:
• Uses the gravity model and capacityrestrained assignment
• But, calculates the number of
development trips consistent with ITE
Replicating ITE Results in FSUTMS
(For the proposed development’s TAZs)
• Two alternative methods:
– Special Generator method
• uses model’s SPECGEN input file
• requires running the model and adjusting several
times
– Link Distribution Percentages method
• uses conventional ZONEDATA input file
• single model run indicates percentage of total
external development traffic traveling on each link
Special Generator Method
• Uses SPECGEN input file
• Requires several assumptions to factor the
development’s ITE-generated trips:
– Vehicle-trips to person-trips
(using vehicle occupancy factors)
– Percent trips by purpose (work, shopping,
social-recreational, non-home-based, etc)
– Percent productions versus attractions
ey Logo / Arena in Miami
Special Generator Method
• Requires trial-and-error attempts to match
FSUTMS to ITE
– Check final trip table after FSUTMS trip attraction
balancing and vehicle occupancy factoring
– Compare to ITE calculations
– Adjust SPECGEN inputs and rerun model until traffic
loadings to/from development zones match
development’s ITE external trip generation
Link Distribution Percentages Method
• Insert dwelling unit and employment estimates
into conventional ZONEDATA input file
– Requires converting square feet to
industrial, commercial, and service
employment
Employees per 1,000 sq ft–Typical Rates
Office: 4
Industrial: 2
Hospital: 3
Warehousing: 1
Retail <200KSF: 3
Hotel: 1 per room
Large-scale retail: 2
• Execute full FSUTMS model run
Source: ITE, ULI
Link Distribution Percentages Method
• For each roadway segment, calculate
development traffic percentage:
Development traffic on link
.
= Link %
Total external generation of site (modeled)

Apply ITE total external generation
to link percentages
Development trips for each roadway link =
Link % x Total External ITE Generation
Link Distribution Percentages Method
ITE-Generated
Development Trip Loadings
FSUTMS Output
(Development Trips)
External
ITE = 15,000
10,000
3,000
30%
15,000
7,000
70%
Distribution
Factors
(manually calculated)
15,000
x 30%
4,500
15,000
x 70%
10,500
Link Distribution Percentages Method Benefits
• Easier application
• Generally similar results as SPECGEN
method
• If development’s land use quantities
change slightly, no need to rerun model
– CAUTION: Significant land use changes
may affect distribution patterns, requiring
a new model run
FSUTMS Highway Assignment
• Based on a capacity-restrained,
equilibrium assignment routine
• Capacity restraint accounts for route diversion
that occurs when drivers encounter congestion
– FSUTMS decreases speeds on congested
roadways after each iteration of the
assignment, until:
• Equilibrium is achieved when all trips in the
MPO area have found the least congested,
shortest-time path to their destination
FSUTMS Site Impact Analysis
HIGHWAY ASSIGNMENT
• Uses the Selected Zone analysis tool
– Single assignment tracks total trips and
development trips throughout the capacityrestrained roadway network
– Background traffic = Total minus
development trips
Selected Zone Analysis Procedures
• Input proposed development’s land use into
zonal data
• Run FSUTMS
• Display traffic that enters/exits development
zone(s) on the loaded network using the traffic
assignment path file
• Save development traffic as a new link attribute
for further analysis
• Check for reasonableness
Using MOCF
Model Output Conversion Factor
• Theory behind use
– Land use inputs reflect peak-season
occupancy rates
– Base-year traffic counts used during validation
represent peak-season volumes
– Therefore, traffic volumes calculated by the
model reflect peak-season weekday average
daily traffic (PSWADT).
MOCF
• AADT needed to calculate peak hour traffic volume:
– AADT x K100 = 100th Highest Hourly Volume
• MOCF converts PSWADT to AADT
(always reduces model output volumes)
• MOCF source: Florida Traffic Information disc Peak
Season Factor reports
• CAUTION: Use MOCF
for appropriate part of
county
(e.g., tourist area vs. rural);
do not reduce
development trips
Internal Trips & FSUTMS
Q .
Is FSUTMS the best method for
estimating the number of internal trips?
A .
FSUTMS is only a tool that may help in a manual
determination of internal trips.
Caution: Size (land area) of TAZs and length of centroid
connectors are the prime determinants of intrazonal trips
in FSUTMS. (Longer centroid link = more intrazonal trips.)
The model does not consider whether the specific land
use types in the proposed development are compatible
and thus, conducive to internal trip making.
Selected Zone
Versus “With & Without”
• Selected Zone Analysis
– Single model run with total trips and development
trips
– Development trips are tracked within the total trip
assignment paths
– Retains the capacity-restraint trip diversion that
occurs in the assignment of total trips
Show with
Show without
Selected Zone
Versus “With & Without”
• “With & Without” Methodology
– Two separate model runs, one with proposed
development in place, the other with development’s
zonal data zeroed out
– Link volumes for “without” run subtracted from “with”
run, yielding net impact of development on each
roadway link
Selected Zone
Versus “With & Without”
• “With & Without” methodology presents a
problem:
– Equilibrium highway assignment capacity-restraint
equation diverts trips, often resulting
in virtually no change in traffic volumes
– Developer: “So, what! Diversion occurs in the real
world. I should only be required to mitigate for net
impacts of the development.”
“With & Without” Method
FSUTMS Total Trips
(With Development)
“Net” Impact???
10,000
50,000
FSUTMS Total Trips
(Without Development)
0
48,000
10,000
45,000
48,000
50,000
- 48,000
2,000
45,000
-48,000
- 3,000
or
Ø
Selected Zone Versus
“With & Without”
• Court’s Ruling:
– DRI process requires accounting for ALL trips caused by
development, NOT net impact resulting from displacing
existing trips to other roadways
– Rationale: if all developers used the argument that trips
are diverted and net impact is negligible, no developer
would be responsible for mitigation
Westinghouse Gateway Communities, et al. v. Lee
County Board of County Commissioners Case
Nos: 90-2636DRI and 90-2638DRI, Jan 14, 1991.
“With & Without” Method
FSUTMS Total Trips
(With Development)
“Net” Impact???
10,000
50,000
FSUTMS Total Trips
(Without Development)
0
48,000
10,000
45,000
48,000
50,000
- 48,000
2,000
45,000
-48,000
- 3,000
or
Ø
Distribution/Assignment Summary
• Definition: Identifying traffic volumes
on each roadway link
– Total traffic
– Development traffic
– At future analysis years
• Next step after assignment: Determine
if any links are:
– Operating at adverse LOS
AND
– Carrying a significant volume of development
traffic
FSUTMS Resources
• www.fsutmsonline.net
– Model Task Force
– FSUTMS training workshops
– Modeling newsletter
– Documentation
– Technical support