Transcript Document

CE 451 - Urban Transportation Planning and Management
Iowa State University
Calibration and Adjustment Techniques, Part 2
Source: Calibration and Adjustment of System Planning Models
5.0 Trip Distribution
5.1 Mean Trip Length
- recall: shape of curve affects trip length distribution
-See below for effect of changing friction factors
F
Curve
trips
link vols.
F
tt
more long trips
more short trips
internal vols.
-varying trip length has a big
impact on assigned volumes
-portions of a friction factor
table can be adjusted (more
flexible than adjusting
equations)
5.2 Estimate Trip Length
-compare average trip
lengths (in minutes) by
purpose to:
HBW
HBSR
HBSh
NHB
t = 0.98 x p.19
t = 2.18 x p.12
t = 8.1
t = 0.63 x p.20
where p is population
SR = social/recreation
Sh = shopping
From Minimum Travel Demand Model
Calibration and Validation Guidelines for the
State of TN
Source: Virginia Travel Demand Modeling Policies and Procedures Manual
Source: Virginia Travel Demand Modeling Policies and Procedures Manual
Source: Virginia Travel Demand Modeling Policies and Procedures Manual
5.3 Employment Distribution
Problems (large cities, mostly)
problem: match low income
households with low income jobs
solution #1: disaggregate trip
purposes by income quartile
solution #2: use k-factors (trial and
error) … yuk
5.4 Special Treatment, other trip
purposes
- schools (ignore if small %?)
- trucks (calibrate with externals?)
-Taxi
normally, distortions are insignificant
6.0 Traffic assignment
6.1 All or nothing
- adjusting link speeds will change assigned volumes
- initial speeds should be set to LOS C speeds (0.87
x free flow speeds)
6.2 capacity restraint
- volume = f(time)
- final volume is average of all
iterations or later iterations can be
weighted more heavily
- adjust free flow time or c (capacity)
to change volumes
IF…
Link
Capacity
THEN…
Speed
Travel
Time
Assigned
Volume
6.2.1 definition of capacity
design: LOS C (0.87c)
ultimate: LOS E (1.00c)
parameters differ depending on
definition of capacity …
if defined as LOS C, 0.15(v/c)4
if defined as LOS E, 0.80(v/c)4 (see
HCM)
7.0 Transit Ridership
- for small/medium cities, may not have to build a transit
network
- If not using a transit network, can use the following
method (if trip generation includes transit trips):
1. increase auto occupancy by transit percentage (e.g. if
auto occupancy is 1.05, then change to 1.05 x 1.38 = 1.45)
if transit percentage is 38%
2. decrease trip production or attraction rates (one of
them only, then balance) … if you use productions, can vary
mode split by income class
3. modify productions or attractions by zone
- get data from transit company
- adjust socioeconomic data or make direct P/A
adjustments
8. External stations
- externals have no socioeconomic data
- Ps and As are prepared by matching ground counts
- I/E treated with the gravity model
-E/E
- compare with Table 11 below
9. System vs. local checks
check
1. system wide (screenlines)
2. major movements (cutlines)
3. links
if all screenlines are high or low, vary
- auto occupancy
- trip generation rates
- trip lengths
- intrazonal times - all zones
- socioeconomic data - all zones
if corridor volumes are high or
low, vary (for zones affecting
corridor…)
- auto occupancy
- trip generation rates
- intrazonal travel times
- land use
- centroid connectors
- intersection penalties
if links are high/low, vary
- speed
- intersection penalty
- centroid locations
- special generators
- local network configuration
10. Expected/Required accuracy
 We are concerned about errors that would require a
design change (e.g. number of lanes)
 Note that ground counts also contain error
 Perfectly calibrated models produce link estimates with
1/3 above the standard error in ground counts and 2/3
below the standard error.
 Need ground counts for 65% of freeways and arterials,
and a good sample from other facilities
From Minimum Travel Demand Model Calibration and
Validation Guidelines for the State of TN
10. Expected/Required accuracy (cont.)
 The correlation coefficient should be greater than .88
 VMT estimate (region-wide) should be within 5% (take care
to compare same roads in systems)
 VMT/person should be 17-24 for large areas, 10-16 for
smaller areas (see also Table A7, next page)
 VMT/household should be 40-60 for large areas, 30-40 for
smaller areas
From CTRE Employment Data Project:
From Minimum Travel Demand Model
Calibration and Validation Guidelines for the
State of TN
Source: Virginia Travel Demand Modeling Policies and Procedures Manual
From Minimum Travel Demand Model Calibration and
Validation Guidelines for the State of TN
From Minimum Travel Demand Model Calibration and
Validation Guidelines for the State of TN
Source: Virginia Travel Demand Modeling Policies and Procedures Manual
From Minimum Travel Demand Model Calibration and
Validation Guidelines for the State of TN
12. TROUBLE SHOOTING