DTA Anyway Peer Review Panel

SAN FRANCISCO COUNTY TRANSPORTATION AUTHORITY

DTA Anyway Peer Review Panel

Traffic Flow Model Parameters Estimation

July 25th, 2012


2

Problem Statement

• Do …

• default traffic flow settings reflect local conditions?

• traffic flow parameter differences between facility and area classifications accurately reflect differences?

• the slopes of San Francisco’s famously hilly streets affect traffic flow conditions (in a meaningful way)?

Source: SF Citizen - http://sfcitizen.com/blog/tag/cable-cars/


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Dynameq Representation of Traffic Flow

• Triangular fundamental diagram

Flow (q)

Density (k)

Critical Density (kc)

Saturation Flow Rate

1FFS

1


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Dynameq Representation of Traffic Flow

• Parameters• Free-flow speed (FFS) - mph• Saturation flow rate (Qs) - pcuplph

• Inverse of saturation flow headway (H) - sec

• Response time (RT) - sec• Backwards wave speed (BWS) - mph• Jam density (Kj) - pcuplpm

• Inverse of effective car length (EL) - ft


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• Challenges:• Triangular simplification forces

compromises• Uninterrupted “flow-density”,

“speed-flow”, or “speed-density” data is required (not available for all the network)

• Use FFS, H, EL, and RT instead• Lack of data for RT and EL

Relating “Real” Speed-Flow toDynameq Parameters


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Data Resources

• Existing resources• Caltrans PeMS (for

freeways)• SFMTA speed surveys

• New resources• Traffic flow observations


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Caltrans Performance Measurement System(PeMS)• Caltrans freeway sensors• Real-time and time-series traffic flow

data• Counts, speeds, occupancy, etc.

• San Francisco coverage:• 15 sensors• US-101, I-280, and I-80

• Resolution of 5-minute period, individual lanes

• Reference period - May 2012.


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Caltrans Performance Measurement System(PeMS)

San Francisco sensor locations


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Caltrans Performance Measurement System(PeMS)• Analysis

• Generated hourly flow-density scatter plot• Piece-wise linear (triangular) curves were

extracted from flow-density scatter plots for 59 freeway lanes in San Francisco

• Non- triangular trends were excluded• 29 removed, 30 retained

• Used for estimation of:• Free-flow speed • Backwards wave speed• Saturation flow rate


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Caltrans Performance Measurement System(PeMS)Curve fitting for flow-density scatter plots

Fixed points

Kj=220

Flow

Density

FFS BWS


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Caltrans Performance Measurement System(PeMS)Curve fitting – triangular plot example 1

Flow vs. density scatter points, Highway 280, Lane 3 at Mission St.


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Flow vs. density scatter points, Highway 280, Lane 3 at Alemany Blvd.

Curve fitting – triangular plot example 2



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Flow vs. density scatter points, Highway 280, Lane 4 at Vermont St.

Non-triangular density-flow plot example 1


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Flow vs. density scatter points, Highway 101, Lane 3 at 25th St.

Non-triangular density-flow plots example 2


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San Francisco Municipal Transportation Agency (SFMTA) Speed Surveys• Speed data collected for speed limit determination

• 500+ locations (~200 used for speed param. adjust)

• Observation facility types

• Arterial (most observations)

• Local/Collector (limited observations)

• Collection period from 2004 to 2012

• Methodology

• 4 second headway requirement

• Off-peak hours during normal weather conditions

• 50th and 85th percentile speeds are used.


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San Francisco Municipal Transportation Agency (SFMTA) Speed SurveysSpeed survey locations Area Type

•AT=0 (reg’l core) 24 locs

•AT=1 (CBD) 105

•AT=2 (urban biz) 164

•AT=3 (urban) 212

Facility Type

•Super arterial 69 locs

•Major arterial 187

•Minor arterial 134

•Collector 56

•Local 57

Slope

•m < -8% 20 locs

•-8% < m < 8% 467

•m > 8% 20


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San Francisco Municipal Transportation Agency (SFMTA) Speed Surveys50th Percentile Speeds 85th Percentile

SpeedsAT \ FT (mph)

Local / Collect

orArterial

Reg’l Core NA 26

CBD 25 26

Urban Biz 26 30

Urban 27 32

Suburban 32 NA

AT \ FT (mph)

Local / Collect

orArterial

Reg’l Core NA 30

CBD 30 31

Urban Biz 31 34

Urban 32 36

Suburban 37 NA


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DTA Project Traffic Flow Survey

Survey Design•Objective

• Record saturation flow rate, backwards wave speed, and jam density on local, collector, and arterial streets of varying slope

•Methodology• Observe jam density from queue

spacing

• Observe backwards wave and saturation flow during queue release


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Time

Saturation Flow

RT

EL

Space

Backward

Shockwave

HRed Light

Queue Dissipation


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• Collected data - vehicles• Vehicle type (car, truck, bus,

motorcycle)• Queue positions• Front bumper distance from stop bar• Time when vehicle begins to move• Time when vehicle passes stop bar

• Collected data - vehicles• Approximate slope of street• Lane width• Movement permission by lane


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• Ideal survey conditions• Single vehicle type – passenger cars• Consistently long queues (≥7

vehicles)• Smooth queue dissipation – no

downstream backup• Absence of conflicts

• Turning movement pedestrian conflicts

• Lane changes


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• Challenges

• Queue lengths• Low facility type (local /collector) streets

lack adequate volume• Steep streets often lack heavy volumes• Need flaw in signal progression for queue

formation• Congestion

• Promising locations suffer from congestion• Conflicts

• Need multiple lanes in order to observe through-movement-only lanes


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• Challenges (continued)• Vehicles do not cooperate !

• Tightening gaps after stoppingWhat is the effective length?

• Inching forward at start of green phase before car in front accelerates

What is the response time?

• Distracted drivers don’t notice the car in front is gone

What is the headway?


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#

Dir. Observ. Street

Cross Street

# of Lane

s FT AT

Measur

ed Grade

Network Grade

# of Queue

s

1 NB

Leavenwort

h O'Farrell 3 Minor ArtReg’l Core 11.2% 11.1% 28

2 NB

Leavenwort

h Geary 3 Minor ArtReg’l Core 7.9% 7.4% 20

3 EB Golden Gate Franklin 3 Major Art CBD Flat Flat 42

4 EB O'Farrell Van Ness 2 Super Art CBD -10% -10.3% 42

5 NB Kearny Bush 3 Major ArtReg’l Core Flat Flat 42

6 WB Fell Buchanan 4 Super Art CBD 5.2% 9.7% 42

7 SB Hyde Post 3 Minor ArtReg’l Core -7.9% -11.2% 42

8 NB Taylor Post 3 CollectorReg’l Core 8.5% 8.3% 20

9 NB Van Ness Golden Gate 3 Major Art

Reg’l Core Flat Flat 42

Survey Locations


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Parameter

Slope Sample Size

Mean St. Dev. Error (90%)

EffectiveLength

(Ft.)

Down 756 24.54 5.41 0.32

Flat 1,134 24.05 5.14 0.25

Up 990 24.32 5.63 0.29

Headway(Sec.)

Down 756 2.38 0.73 0.044

Flat 1,125 2.57 0.91 0.044

Up 558 2.72 0.78 0.054

ResponseTime(Sec.)

Down 747 1.09 0.39 0.023

Flat 378 1.37 0.45 0.038

Up 558 1.38 0.90 0.062

Collected Data Summary Statistics


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Parameter Data Sources

Data sources for parameters by facility type

Param.

FT

Free-flow Speed

Saturation Flow Response Time Jam Density

Freeway FTs

PeMS PeMS PeMSInferred from CBD

arterials

Arterial FTs

SFMTA speed surveys

CBD saturation headway

observations

CBD queue dissipation

observations

CBD arterial queue length observations

Local & Collector FTs

Limited SFMTA speed surveys

& supplemental observations

Mostly inferred from CBD arterials



Red text = data limitations


27

Parameter Comparison

• Comparison of original / default parameters to updated parameters (no slope variation)• Free-flow speeds increase in some ATs• Response time is slower• Jam density is lower• Saturation flow rates are lower by 100-

300 pcuplph• Trade-off between saturation flow and

response time• Matching either param. to data throws other off

• Compromise is slightly high flow, slightly slow response


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Freeway

Area Type SettingsFree Flow

Speed (mph)

Saturation Flow

(pcuphpl)

Effective Length (ft,

PCU)

Response Time

(seconds)

Regional Core

(AT=0)

Original 45 2307 20.5 1.25

Updated 60 2185 24.0 1.37

CBD(AT=1)

Original 50 2353 20.5 1.25

Updated 65 2213 24.0 1.37

Urban Business(AT=2)

Original 55 2393 20.5 1.25

Updated 65 2213 24.0 1.37

Urban(AT=3)

Original 60 2428 20.5 1.25

Updated 65 2213 24.0 1.37


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Major Arterial


Speed (mph)

Saturation Flow

(pcuphpl)


PCU)

Response Time

(seconds)

Regional Core

(AT=0)

Original 25 1990 20.5 1.25

Updated 30 1760 24.0 1.5

CBD(AT=1)

Original 30 2098 20.5 1.25

Updated 30 1760 24.0 1.5


Original 35 2183 20.5 1.25

Updated 35 1830 24.0 1.5

Urban(AT=3)

Original 35 2183 20.5 1.25

Updated 35 1830 24.0 1.5


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Local Street


Speed (mph)

Saturation Flow

(pcuphpl)


PCU)

Response Time

(seconds)

Regional Core

(AT=0)

Original 20 1847 20.5 1.25

Updated 25 1671 24.0 1.5

CBD(AT=1)

Original 25 1990 20.5 1.25

Updated 25 1671 24.0 1.5


Original 30 2098 20.5 1.25

Updated 30 1760 24.0 1.5

Urban(AT=3)

Original 30 2098 20.5 1.25

Updated 30 1760 24.0 1.5


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Parameter ComparisonIntroduction of Slope Variation

• Slope-specific parameters not yet implemented in calibration model runs, but will be soon

• Modify response time and saturation flow

• Hold speed and effective length constant

• Do not modify parameters on freeway FTs• Generally less steep• Slope data does not reflect skyway

and underpass elevation


32

Parameter ComparisonIntroduction of Slope VariationExample: Major arterial in CBD area typeUniform parameters: FFS = 30 mph, EL = 24 ft


The End

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