Source: NHI course on Travel Demand Forecasting (152054A)

Session 11: Model Calibration, Validation, and Reasonableness

Checks

Objectives:

• Identify and interpret trends affecting travel demand • Explain difference between calibration and validation• Identify critical reasonableness checks– socioeconomic– travel survey – network – trip generation – mode split – trip assignment

Terminology• Model Calibration• Model Validation– Reasonableness checks– Sensitivity checks

• Special generators• Screen lines (some modelers do not think this

is important)

Is the model sensitive to policy options?

• Not enough attention on model evaluation and reasonableness checks

• Checks performed after each step– reduces error propagation

Errors can also “cancel”

Key Concepts

Planner responsibilities• Actively involve all participants

– Modelers– Planners– Decision makers– Public

• Fairly present all alternatives– Timely– Unbiased

• Identify (clearly) the decision making process– Who, when, and how– Allows input from all interested groups

• You must rely on the TDM– Therefore, must be validated– Accurate and easy to understand (documented)

• Planners should monitor the following trends:– Demographics– Composition of the labor force– Immigration and emigration– Regional economic development– Modal shares– Vehicle occupancy– Average trip length– Freight transport

• Are trends consistent with assumptions made in the modeling process?

Must be aware of trends to ensure reasonable forecasts

Trends Affecting Travel Demand

Information Requirements for Validation and Reasonableness

• Demographics and employment• Highway and transit networks• Model specification• Base year survey• Base year traffic counts

Sources of Error

• Coding• Sampling• Computation (if done by

hand)• Specification• Data Transfer• Data aggregation

Improper structure of model, e.g., wrong variables

Scrutinize these characteristics: • Data requirements• Hardware requirements• Logic of structure and conceptual appeal • Ease of calibration • Effectiveness of the model (accuracy, sensitivity)• Flexibility in application• Types of available outputs• Operational costs• Experience and successes to date• Public or private domain availability• Compatibility with other models and model types

How do you judge a model/recommend improvement?

Reasonable?Methodology?

Source?

Current?Reasonable

?

Complete?Level of Detail?

Sensitive?Documentation of

calibration?Valid for base

year?

Evaluation and Reasonableness Checks Overview

Transportation Transportation systemsystem

(supply)(supply)►Network DataNetwork Data

Number and Number and location of location of

households and households and employment employment

(demand)(demand)►Socioeconomic Socioeconomic

DataData

TDFTDF►Model SpecificationModel Specification►Model validation and Model validation and calibrationcalibration

Travel Travel

survey datasurvey data

Transportation Transportation systemsystem

performanceperformance

Model Calibration and Validation

Model CalibrationModel Calibration

Model ValidationModel Validation

Model ApplicationModel Application

Feedb

ack

Loop

“Transportation Conformity Guidelines” (Air Quality) require model validated < 10 years ago

Model Validation

• Validation of new model– Model applied to complete model chain– Base year model compared to observed travel– Judgment as to model suitability, return to calibration

if not• Validation of a previously calibrated model– Compare to a new base year, with new …

• SE data• Special gen.• Network• Counts

Socioeconomic Data: Check Reasonableness

• reviewSource for estimates and forecasts• Population and household size trends (graph 1950 to present

and check trend)• Household income trends (graph as far back as this goes …

1990?)• Check dollar values used in forecast (use constant dollars)• If used, check trend of automotive availability (S curve?)• Check distribution of employment by type (basic, retail,

service) over time• Plot and check trend of employees per household and per

capita … rate of increase is decreasing• Check future household and employment changes by zone

Trav

el not

sensit

ive to

fuel

price?

http://www.eia.doe.gov/oiaf/aeo/pdf/trend_4.pdf

Travel Survey Data Reasonableness Checks

• Determine source of travel survey data – Types of survey conducted– Year of survey

• If no survey (borrowed)– Check source of trip rates, lengths, TLFD– Is area similar

• Geographic area?• pop/HH/empl. characteristics?• Urban density and trans system?

• Compare to similar regions and to same region in earlier times:

– Person trip rates by trip purpose– Mean trip lengths by trip purpose

• HBW longest? HBO shortest?– TLFDs by trip purpose

Network Data Reasonableness Checks

• Check Trees for 2-3 major attractions• Check coded facility types – how used (BPR?)?• Verify speed and capacity look-up table (what

LOS used for capacity?)• Significant transportation projects – narrative

included? Still viable?• Consistency with MTP• Plot (facility types, # lanes,

speeds, area types) to detect coding errors

Trip Generation Reasonableness Checks

• Examine trip production and attraction models– Form?– sensitivity?

• Examine trip purposes used• External-through and external-local trips – how

modeled?• Truck trips – how modeled?• Person trip or vehicle trip rates used?• P&A balance (0.9-1.1 ok)• Special generators (check, and be consistent in

future model)

Trip Generation CalibrationTypical Values

• Person trips per household: 8.5 to 10.5• HBW person trips per household: 1.7 to 2.3• HBO person trips per household: 3.5 to 4.8• NHB person trips per household: 1.7 to 2.9• HBW trips: 18% to 27% of all trips• HBO trips: 47% to 54% of all trips• NHB trips: 22% to 31% of all trips

Scale survey for participation (relative participation)

Note: each

income class is

a purpos

e!

TRIP PURPOSES Scaling Factor

HBW low income 0.795

HBW low-middle income 0.823

HBW middle income 0.861

HBW upper middle income 0.908

HBW high income 0.936

HB elementary school 0.733

HB high school 1.991

HB university 0.895

HB shopping 0.698

HB social-recreation 0.945

HB other 0.875

NHB work-related 0.858

NHB other 0.820

Truck 0.985

Internal-external 0.591

Trip Generation Calibration

Colorado Springs 1996 Travel Demand Model Calibration

Trip Generation CalibrationReasonableness checks – compare to other cities, check

future trends• Population 503,345• Households 201,116• Average Household Size 2.50• Basic employment 76,795 (33%)• Retail employment 50,465 (24%)• Service employment 101,697 (43%)• Military employment 42,800• Population per employee 1.81• Person trips per person 4.26• Person trips per household 10.65• HBW attractions per employee 1.44• HBW productions per household 1.74• HB shopping attractions per retail employee 5.99

Colorado Springs 1996 Travel Demand Model Calibration

Trip Distribution Reasonableness Checks

Examine …• Mean trip length (increasing or decreasing?)• TLFDs• Treatment of friction factors (same?)• Treatment of terminal times (logic?)• Treatment of K factors• Comparison with JTW trip length• Comparison with JTW sector interchange volumes or

percentages.

Calibrate

friction factors

1st iteratio

n

Travel TimesTravel TimesRanges from Ranges from SkimsSkims

Observed Trip Observed Trip Expanded from Expanded from SurveysSurveys

Input Input Friction Friction FactorsFactors

Gravity Gravity Model TripsModel Trips

Adjustment Adjustment FactorFactorObservedObservedGravity Gravity ModelModel

New Friction New Friction FactorsFactorsFriction Friction AdjustmentAdjustmentFactor x Friction Factor x Friction FactorFactor

2.52.5 7,1007,100 30.030.0 8,2008,200 0.870.87 25.9825.98

5.05.0 14,95014,950 2.502.50 16,30016,300 0.920.92 2.292.29

7.57.5 17,85017,850 1.801.80 19,25019,250 0.930.93 1.671.67

10.010.0 16,00016,000 1.501.50 19,10019,100 0.840.84 1.261.26

12.512.5 15,50015,500 1.201.20 17,10017,100 0.910.91 1.091.09

15.015.0 15,90015,900 1.001.00 12,30012,300 1.291.29 1.291.29

17.517.5 16,40016,400 0.950.95 18,00018,000 0.910.91 0.870.87

20.020.0 15,15015,150 0.900.90 14,30014,300 1.061.06 0.950.95

22.522.5 13,50013,500 0.850.85 11,90011,900 1.131.13 0.960.96

25.025.0 11,00011,000 0.800.80 9,2509,250 1.191.19 0.950.95

27.527.5 9,5009,500 0.750.75 8,1008,100 1.171.17 0.880.88

30.030.0 9,1009,100 0.700.70 6,1006,100 1.491.49 1.041.04

32.532.5 5,7005,700 0.650.65 4,9004,900 1.161.16 0.760.76

…… …… …… …… …… ……

Calibrating a Gravity ModelAdjusting Friction Factors

2nd iteratio

n

Commute Length in Commute Length in MinutesMinutes PercentPercent

Journey-to-WorkJourney-to-WorkFlowsFlows PercentPercent

< 15< 15 27.8727.87 Central-CentralCentral-CentralCountyCounty

31.4931.49

15-2915-29 41.6341.63 Central-SuburbanCentral-SuburbanCountyCounty

7.487.48

30-3930-39 17.0417.04 Suburban-Central Suburban-Central CountyCounty

15.1315.13

40-5940-59 7.707.70 Within Suburban Within Suburban CountyCounty

32.9832.98

>60>60 3.003.00 To Other Suburban To Other Suburban CountyCounty

10.8110.81

Mean 21.44Mean 21.44 Work out of areaWork out of area 2.112.11

Trip Distribution Calibration and Validation

• Check modeled vs. household survey TLFD and mean trip lengths• Get HBW area-to-area flows from JTW

HBW 1990 JTW TLFD and Area-to-Area Flows for Kansas City

Mode Split Reasonableness Checks

• Automobile occupancy factors by trip purpose used?• Basis? • Constant?

• Mode split model? • Form?• Variables included in the utility functions?

• Coefficients logical?• Value of time assumptions• Parking cost assumptions

• How do mode shares change over time?• Mode share comparisons with other cities

• Experienced planning consultant required …– Form of LOGIT model– Variables included in utility functions– Calibration of coefficients for utility function variables– Testing for IIA properties– Analysis of household survey data– Analysis of on-board transit survey data

• Calibration tasks we can do:• Compare highway and transit trips

• Total• By purpose

• Compare Ridership by route• CBD cordon line survey (if bus service is downtown only)

Mode Split Calibration and Validation

• All-or-nothing assignment • study effect of increasing capacity• Compare to Equilibrium assignment

• Check volume delay equation (BPR parameters)• Compare

• screen line volumes• Cut line volumes

• Time-of-day assignments?• Source of factors• Peak spreading used for future?• If not, conversion factors source?

(peak hour to 24-hour) • Local VMT (% assigned to

intrazonals and centroid connectors All or

Nothing

Equil

ibrium

Trip Assignment Reasonableness Checks

Assignment

calibration

performed last

Trip Assignment Calibration and Validation

Overall VMT or VHT check• 40 to 60 miles per day per HH in large metro areas• 30 to 40 miles per day per HH in medium metro• +/- 10% OK on screen lines• Sign is important

Compute by …- volume group- facility type- transit assignments- time of day

Other Factors Impacting Forecasted Travel Demand (use your noodle)

• Can be implied in travel surveys (but not explicit)– Telecommuting– Flexible work hours– HB business

• How to account for …– Aging population– Internet shopping– Roadway congestion (will it affect generation in the future)– New modes