travel time and reliability: is data quality a showstopper? the georgia navigator experience...
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Travel Time and Reliability: Is Data Quality
a Showstopper?
The Georgia Navigator ExperienceAngshuman Guin
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