Travel Time and Reliability: Is Data Quality

a Showstopper?

The Georgia Navigator ExperienceAngshuman Guin

URS Corporationangshuman_guin@urscorp.com

ITSA Phoenix

May, 2005

Overview

• NaviGAtor Travel Times

• Data Failure Issues

• Remedial Measures– Transportation Sensor System (TSS)– Maintenance Management Plan (MMP)

• Key Stations• VDS Quality Assurance

Georgia NaviGAtor’s Video Detection System (VDS)

1,361 Fixed Black & White Cameras

Spaced Every 1/3 mile on Freeways

Continuous Speed / Volume/ Occupancy Data

Generates Travel Times for CMS

Navigator ATMS Archive

• Data Attributes– Volume / Count – Average Speed (per 15 minutes per lane)

– Lane Occupancy (per 15 minutes per lane)

• Frequency: 20 second data aggregated to 15 minutes data in archive

• Per Lane• Bi-directional• Mainline / Ramps

Travel Times on Website

Travel Time Determination

• Dynamic Message Sign

– Trip section is comprised of 2 Zones– Each Zone is comprised of 2 Sub-Zones– Each Sub-Zone is comprised of several Stations

x

DMS

Destination

Direction of Travel

ZONE 1

ZONE 2

Subzone 1-1

Subzone 1-2

Subzone 2-1

Subzone 2-2

Travel Time Determination cont.• Dynamic Message Signs cont.

– 20 Second Station average speeds are aggregated into 1 minute Station, Sub-Zone and Zone average speeds– 1 Minute Zone average speeds are categorized as:

• Moving Very Well (55 + mph)• Moving Well (40 – 55 mph)• Moving Slowly (30 – 40 mph)• Moving Very Slowly (< 30 mph)

Travel Time Determination cont.

• Dynamic Message Signs cont.

– 16 Travel Time messages are created in the message library for each DMS and displayed as traffic conditions change according to the matrix below

Message: Travel Time = 5 - 6 min

Message: Travel Time = 16+ min

Message: Travel Time = 10 - 11 min

Travel Times as a Performance Measure

Travel Time Variability

Data Failure Issues

Existing Archived Data Flow

Remedial Measures

• Data Archive– Architecture (Transportation Sensor

System – TSS)– Data sample

• Data Collection (Maintenance Management Plan)– Hardware– Software

TSS Process

- Server Administration

- Communication

New Navigator Archive• XML Format• 5-Minute intervals

instead of 15-Minute• Truck percentages• Filtering of data to

eliminate bogus data• Meta-data information

Transportation Sensor System (TSS) Attributes

• Detector ID (integer)

• StartTime (datetime)

• Duration (integer, seconds)

• Total-volume (integer, 0+)

• Percent-trucks (float, 0.00 - 1.00)

• Lane Occupancy (float, 0.00 - 1.00)

• Average Speed (float, 0.0+)

• Std. Dev. Lane Occupancy (float, 0.00 - 1.00)

• Std. Dev. Average Speed (float, 0.0+)

• VALIDITY (integer 1 - 100)

– Percentage of valid samples in 5-minute aggregate

• AVAILABILITY (integer 1 -100)

– Percentage of available samples in 5-minute aggregate

Data Quality Measures: Availability and Validity

 where:

nuseable values = the number of data samples with values present in the aggregate

ntotal expected = the total number of data samples expected for the aggregate

where:

nvalid = the number of data samples with values meeting the validity criteria

ntotal expected = the total number of data samples expected for the aggregate

ectedtotal

valuesuseable

n

nA

exp

ectedtotal

valid

n

nL

exp

Validity Criteria

Attribute Term DefinitionVolume (veh/h) Too High > 3600

High >= 2700Nearly Zero < 360Zero 0

Speed (MPH) Too High > 100Low < 20Nearly Zero <= 10Zero 0

Occupancy (%) Too High > 100High > 50Medium >= 10 & < 30Low < 10Nearly Zero < 2Zero 0

Volume Speed Occupancy

Zero Zero ZeroAny Low MediumHigh Any Low or HighNearly Zero

Nearly Zero

Not Nearly Zero

Too High Any AnyAny Any Too HighAny Too High Any

(conditions must be true for all three conditions to be considered invalid)

1

2

3

4

5

6

7

5

4

2

3 36

7

Lane Occupancy ---->

Volume

Maintenance Management Plan

Hardware Failures Quality Assurance

Maintenance Management Plan

• Data calibration and validation with Ground Truth data

• 1361 VDS Stations (5000+ detectors)

• Key Stations (20+)

• Problematic Detector Indicator Methodology

Key Stations & Priority Stations

Problematic Detector Indicator Methodology

80%

85%

90%

95%

100%

105%

110%

115%

120%

-8 -6 -4 -2 0 2 4 6 8

Stations From Key Station

Err

or

Bo

un

ds

(%)

Upper BoundLower Bound

where:

Ev : expected value for the station volume,

Vk : volume of the key station associated with this station,

KTOD DOW f : key-station adjustment factor for TOD and DOW,

ε : expected error tolerance

)1( fDOWTODkv KVE

Maintenance Management Plan

• Key Stations (20+)– Maintenance not only for failure but also for data

quality

• Data calibration and validation with Ground Truth data– Formalized procedure– Defined sample requirements– Use Hypothesis Testing (paired-t)– Obtain accuracy statistics

Questions?

www.georgia-navigator.com

mynav.georgia-navigator.com