NCHRP 17-45 TES.ppt - Kittelson & Associates,...

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Enhanced Safety Prediction Methodology and Analysis Tool for Freeways and

Interchanges

March 2012

NCHRP Project 17-45

Overview

• Background on HSM

• Project Overview

• Safety Evaluation Overview

• Freeway Predictive Method

• Ramp Predictive Method

• Closure

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Background on HSM

• HSM Purpose

– To provide tools and techniques for quantifying the relationship between infrastructure element presence or dimension on safety

– These tools and techniques can be used to quantify the potential change in safety resulting from a change in geometric design or control feature

– Safety is explicitly described in terms of expected crash frequency and severity

Background on HSM

• History

– 10‐year research program funded by NCHRP and FHWA

– Development guided by TRB’s Task Force for the Development of a Highway Safety Manual

• Renamed Highway Safety Performance committee

– Kittelson & Associates, Inc. was prime contractor responsible for producing the document

– Published by AASHTO

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Background on HSM

• Part C – Predictive Method

– Used to estimate the expected crash frequency and severity for a facility, or portion of a facility

– The estimate can describe existing conditions or a proposed design

– Methods provided in Part C

• Chapter 10 – Rural two‐lane roads

• Chapter 11 – Rural multilane highways

• Chapter 12 – Urban and suburban arterials

Background on HSM

• Part C – Predictive Method

– Application

• Facility is decomposed into individual road segments and intersections called “sites”

• Method is used to evaluate individual sites

• Results are added to describe facility performance

– Performance Measures

• Crash severity (K, A, B, C, and PDO)

• Crash type – Fixed object, parked vehicle, animal, other

– Head on , right angle, rear end, sideswipe, other

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Project Overview

• Objectives

– Develop predictive methods

• Freeways

• Interchanges

– Develop a software tool that implements the methods

– Document in a chapter for a future HSM

– Develop algorithm documentation for IHSDM

Project Overview• Schedule

– Begin: May 2009

– End: August 2012

• Status

– NCHRP Panel has reviewed and approved

– Draft final report and products under review by TRB’s Highway Safety Performance committee

– Final report and products due May 2012

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Safety Evaluation Overview

• Predictive Model

• Predictive Method

• Segmentation Process


• Predictive Model

– Crash frequency, Np = Nspf x CMFlw x CMFsw ... x C

• Model Components

– Safety performance function (SPF), Nspf

– Crash modification factors (CMF), CMFi– Calibration factor, C

Volume Lane Width

Predicted Crash Frequency

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• Safety Performance Function– Crash frequency for segment with base conditions

– Typical: 12 ft lanes, 10 ft outside shoulder, etc.

– Nspf = a × (AADT)b × L

• AADT = annual average daily traffic volume

• L = segment length

– May be sensitive to...• Crash severity: fatal‐and‐inj. (FI), prop.‐damage‐only (PDO)

• Crash type: multiple‐vehicle (MV), single‐vehicle (SV)

• Area type: urban, rural

• Number of lanes

Safety Evaluation Overview• Crash Modification Factor

– Change in crash frequency for a specific change in geometry (or the application of a treatment)

– Adapts SPF to non‐base conditions

– One CMF per design element (e.g., lane width)

– CMF = 1.00 for base condition value

• Project 17‐45 CMFs– 13 CMFs for freeways

– 9 CMFs for ramps

– 11 CMFs for crossroad ramp terminals

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• Example Crash Modification Factor

– Existing 4‐lane freeway

– Base condition

• 12 ft lanes

• CMFbase = 1.00

– Existing roadway

• 11 ft lanes

• CMF11 ft = 1.04

0.90

0.95

1.00

1.05

1.10

10.5 11 11.5 12 12.5 13 13.5 14

Lane Width (ft)

Cra

sh

Mo

difi

catio

n F

act

or


• Predictive Method– 18 steps in HSM predictive method

– Use predictive model to evaluate all sites

– See HSM Part C – Introduction for details

– Severity distribution• HSM Part C – table of typical distribution values (1% K, 5% A, etc.)

• Project 17‐45 ‐ severity distribution functions (SDFs)

• Project 17‐45 Software Implementation– 7 steps

– First 6 steps are the same as in HSM method

– Software automates remaining steps

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• Segmentation Process

– Divide roadway into sites

– Sites

• Homogenous segments

• Intersections

Ren1

Rex3

In

CD2 CD3CD1

Rex5 Ren2

Cro

ssro

adRamp Entrance

Length, Len

Ramp Exit

Length, Lex

Fr1 Fr2 Fr3

PLAN VIEW

Rex4

CD4 CD5


• Definition ‐ Homogeneous Segment

– A homogeneous segment has the same basic character for its full length

• Number of lanes

• Lane width

• Shoulder width

• Median width

• Clear zone width

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• Example Segmentation

– Begin segment at gore point

Lane add

Segment

Median

L

Begin milepost(at gore point)

End milepost

Input Data1. Number of through lanes: 52. Ramp entrance in segment? "Lane Add"3. Ramp exit in segment? "No"4. Type B weave in segment? "No"

Freeway Predictive Method

• Safety Performance Function

• Crash Modification Factors

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Freeway Predictive Method• Safety Performance Function


– Coefficients a and b by...

• Area type

• Number of lanes

• Crash type

• Severity

0

3

6

9

12

15

0 50 100 150 200 250

Average Daily Traffic Demand (1000s), veh/day

FI M

ult

iple

-Ve

hic

le C

ras

h

Fre

qu

en

cy

, cra

sh

es

/yr

1.0-mile segment length, no barrier

4 lanes

6

8

Rural FreewayUrban Freeway

6

10

8


• Crash Modification Factors

– Horizontal curve

– Lane width

– Paved shoulder width

• Inside

• Outside

– Median width

– Barrier

• Median

• Roadside

– High volume (congestion)

– Lane change (ramp related)

– Shoulder rumble strips

– Outside clearance

– Ramp entrance

– Ramp exit

Lr

2'

Tapered Ramp

Lr = ramp entrance length

Lr

2'

Parallel Ramp

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• High Volume CMF

– Proportion of AADT during hours where volume exceeds 1,000 veh/h/ln

0

500

1,000

1,500

2,000

1 3 5 7 9 11 13 15 17 19 21 23

Hour of Day

Ho

url

y V

olu

me

(ve

h/h

/ln)

Route 51, Sacramento Co., Calif. Route 5, Shasta Co., Calif.

– Proportion > 0.0 if

• Hourly volumes continuously high, or

• A few very high peak hours exist

– Use nearest traffic count station data

Phv = 0.89

Phv = 0.0


• High Volume CMF

– Base Condition

• Proportion = 0.0

– Limits

• 0.0 to 1.0

– Notes

• Default values available

1.00

1.05

1.10

1.15

1.20

0.0 0.2 0.4 0.6 0.8 1.0

Proportion AADT during High-Volume Hours

Cra

sh

Mo

dif

ica

tio

n F

ac

tor

Rural 6-lane, proposed

Urban, proposed

Rural 4-lane proposed

Multiple-vehicle crashes

0.94

0.96

0.98

1.00

0.0 0.2 0.4 0.6 0.8 1.0

Proportion AADT during High-Volume Hours

Cra

sh

Mo

dif

ica

tio

n F

ac

tor

Rural 6-lane, proposed

Urban, proposed

Rural 4-lane proposed

Single-vehicle crashes

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Segment

Increasing mile post

Begin milepost

Xb,ent

Xb,ext

End milepost

Xe,ext

Xe,ent

All measurements are to the marked gore point.

AADTb,ent

AADTb,ext

AADTe,ent

AADTe,ext


• Lane Change CMF

– Ramp Location and AADT

– Need for upstream entrances and downstream exits

• Not upstream exits and downstream entrances


• Lane Change CMF

– Base condition

• Ramps > 0.5 mi

– Notes

• Includes adjustment for weaving sections

1.0

1.1

1.2

1.3

0.05 0.15 0.25 0.35 0.45

Distance from Left-Side Gore (x), mi

Cra

sh

Mo

dif

ica

tio

n F

ac

tor

Rural 6-lane freewayRamp AADT = 6,000 veh/day

Segment Length = 0.1 mi

Average CMF for 0.5-mi road section = 1.10

x

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Ramp Predictive Method

• Ramp and C‐D Road Segments

– Ramp configurations

– Safety performance function

– Crash modification factors

• Crossroad Ramp Terminals

– Terminal configurations




• Ramp Configurations

– Segment based, works for all ramp configurations

Buttonhook

Collector-Distributor

Parclo Loop(Non-Free-Flow)

Free-Flow Loop

Outer Connection Direct Connection(used at directional interchanges)

Semi-Direct Connection(used at directional interchanges)

Diamond(Diagonal)

One curve

Two curves

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Ramp Predictive Method• SPF


– Coefficients a and b by...

• Exit, entrance, C‐D road

• Area type

• Number of lanes

• Crash type

• Severity

0.00

0.10

0.20

0.30

0.40

0 4 8 12 16 20 24 28

AADT (1000s of veh/d)

Fa

tal-

an

d-I

nju

ry C

ras

h

Fre

qu

en

cy

(c

ras

he

s/y

r)

1 Lane

2 Lanes

0.2 mile segment length, No barrier, no curves

Rural rampUrban ramp

Entrance Ramp

0.00

0.10

0.20

0.30

0.40

0 4 8 12 16 20 24 28

AADT (1000s of veh/d)

Fa

tal-

an

d-I

nju

ry C

ras

h

Fre

qu

en

cy

(c

ras

he

s/y

r)

1 Lane

0.2 Mile segment length, No barrier, no curves

1 or 2 Lanes

Rural rampUrban ramp

Exit Ramp


• CMFs– Horizontal curve

– Lane width

– Right shoulder width

– Left shoulder width

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• CMFs

– Right side barrier

– Left side barrier

– Lane add, lane drop

– Ramp speed‐change lane

– Weaving section (C‐D road only)

• Supplemental Models

– Ramp curve speed prediction model


• Horizontal Curve CMF

1.0

1.2

1.4

1.6

1.8

2.0

0 500 1000 1500 2000 2500 3000 3500

Curve Radius, ft

Cra

sh

Mo

dif

ica

tio

n F

ac

tor

.

Two-Lane HighwayHarwood et al. (2000)Def. Angle = 30 deg.

Curve speed = 50 mi/h

40 mi/h

20 mi/h

30 mi/h

Urban, Single-Lane Ramp

Yates (1970)

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• Ramp and C‐D Road Segments

– Ramp configurations



• Crossroad Ramp Terminals

– Terminal configurations



Ramp Predictive Method• Ramp Terminal Configurations

– Terminal‐configuration based

– 7 configurations

Crossroad

Ramp

Type: D3ex

Type: D3en

Ramp

Fre

eway

Fre

eway

Ramp

Type: D4

Fre

eway

Diagonal, Exit Diagonal, Entrance

Diagonal, 4‐Leg

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• Ramp Terminal Configurations (continued)

– 7 configurationsRamp

Type: A4

Ramp

Fre

eway

Ramp

Type: B4

Ramp

Fre

eway

Ramp

Type: A2

Fre

eway

Type: B2

Ramp

Fre

eway

4‐quad Parclo A 4‐quad Parclo B

2‐quad Parclo B2‐quad Parclo A

Ramp Predictive Method• Safety Performance Function

– Nspf = a × (AADTxrd)b × (AADTex + AADTen)

d

– AADTxrd = crossroad traffic = (AADTin + AADTout)/2

– AADTen = entrance ramp traffic

– AADTex = exit ramp traffic

– Coefficients a, b, d by...• Configuration

• Type of control (signal, stop)

• Area type

• Number of crossroad lanes

• Severity

Crossroad

Left-Side Ramp Right-Side Ramp

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• SPFs

– D4 Configuration

• One‐way stop

• Signal0.0

0.4

0.8

1.2

1.6

2.0

0 5 10 15 20 25

Crossroad AADT (1000s of veh/day)

Fa

tal-

an

d-l

nju

ry C

ras

h

Fre

qu

en

cy

(cra

sh

es/y

r)

Terminal Type: D4Ramp AADT = 0.32 x Crossroad AADT

4-lane crossroad, 2-lane exit ramp


Urban areaExit ramp right turn: stop controlledAll other CMFs = 1.00

0

5

10

15

20

0 10 20 30 40 50 60

Crossroad AADT (1000s of veh/day)

Fa

tal-

an

d-l

nju

ry C

ras

h

Fre

qu

en

cy (

cra

sh

es

/yr)

Terminal Type: D4Ramp AADT = 0.32 x Crossroad AADT 6-lane crossroad,

2-lane exit ramp

4-lane crossroad,1-lane exit ramp


Exit ramp right turn: signal controlledAll other CMFs = 1.00

Ramp

Type: D4

Fre

eway

Signal

Stop


• CMFs

• Signal or Stop

– Exit ramp capacity

– Crossroad turn lane

• Left turn

• Right turn

– Access point frequency

– Segment length

– Median width

• Signal

– Protected‐only left‐turn phase

– Channelized right turn

• Crossroad

• Exit ramp

– Non‐ramp leg

• Stop

– Skew angle

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• Median Width CMF

– Stop Control

0.0

0.5

1.0

1.5

2.0

2.5

10 20 30 40 50 60

Median Width, ft

Cra

sh

Mo

dif

ica

tio

n F

ac

tor

Ramp Terminal, 14,000 veh/d, proposed

Rural, 4 Legs (Highway, 2010)

Urban, 4 Legs (Highway, 2010)

Ramp Terminal, 5000 veh/d, proposed

Median

Closure• Methods Do Not Address the Following

– Freeways with continuous access HOV lanes

– Freeways with buffer‐separated managed lanes

– Use of safety shoulders as travel lanes

– Ramp meters

– HOV by‐pass lanes on ramps

– Crossroad segments

– Frontage road segments or terminals

Buffer Barrier (ok)

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Closure• Safety Evaluation Scenarios

– Freeway• Interchange spacing

• Weaving section length

• Reallocation of cross section width

– Interchange• System interchange configuration

• Service interchange configuration (parclo, diamond)

– Analysis Scale• Single year analysis or design life analysis

• Freeway system analysis or individual section

Closure

• Implementation Plan

– Practitioner‐Oriented Products

• ISATe software

• ISATe User Manual

• HSM draft chapters

• Training course content

– User Evaluation Procedure

• Detailed evaluation by freeway design experts

• Two, one‐day practitioner workshops

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Closure

• Questions or Comments?

NCHRP 17-45 TES.ppt - Kittelson & Associates,...

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