NCHRP Project 17-70 Development of Roundabout Crash Prediction Models and Methods - PowerPoint Presentation

Slide1

NCHRP Project 17-70Development of Roundabout Crash Prediction Models and Methods

Kittelson

& Associates, Inc.

In association with:

Persaud

and Lyon, Inc.

Write Rhetoric

Slide2

Project ObjectivesDevelop Crash Prediction Models (CPMs) for U.S. roundabouts for planning and design decisions

Specific questions addressed:

How do geometric features – and combinations of features – influence the number and severity of crashes at the roundabout?

How do operational features – and combinations of features – influence the number and severity of crashes at the roundabout?

How do driver learning curves influence the number and severity of crashes at any age roundabout?

CPMs to be included in next edition of the HSM,

SafetyAnalyst

and IHSDMSlide3

Presentation OrganizationData collectedModel development approach

CPMs developed

Driver learning curve findings

ConclusionsSlide4

Data CollectedCrash, traffic and geometric data collectedCA, FL, KS, MI, MN, NC, NY, ON, PA, WA, WI

Intersection-level CPMs – 327 sites

Leg-level CPMs – 150 sites, 534 individual legs

Driver learning curve – 109 sites (FL, MI, NY, WA, WI)Slide5

Model Development ApproachConsistent with HSM chapter C predictive chaptersCrashes predicted with a combination of SPFs and CMFs

Cross-sectional negative binomial regression models developed

Annual crash frequency modeled as a function of traffic volumes and geometric variablesSlide6

CPMs for Planning-Level Analysis

Apply to entire intersection

Developed using only AADT variables and basic geometric variables that would be known at the planning stage

Models are applicable for average conditions of other geometric characteristics

CPMs used early in the project development process

Network screening

Intersection control evaluations

Predict crashes per year for crashes within circulating roadway and on legs and considered related to roundabout geometry or operation

Pedestrian and bicycle crashes not includedSlide7

CPMs for Planning-Level AnalysisRural Roundabouts

N =

exp

a

MAJAADT

b

MINAADT

c

exp

d

×

NUMBERLEGS+e

× CIRCLANES

N = predicted average crash frequency, crashes/

yr

MAJAADT = Total entering AADT on major road

MINAADT = Total entering AADT on minor road

NUMBERLEGS = 1 if a 3-leg roundabout; 0 if 4-legs

CIRCLANES = 1 if a single lane roundabout; 0 if more than 1 circulating lanesSlide8

CPMs for Planning-Level AnalysisRural Roundabouts

N =

exp

a

MAJAADT

b

MINAADT

c

exp

d

×

NUMBERLEGS+e

× CIRCLANES

Severity

a

b

c

d

e

k

TOT

-5.3299

0.3356

0.5142

-0.6854

-0.9375

0.6292

FI

-10.4848

0.7756

0.4239

-1.0080

-0.5506

0.4424

PDO

-5.4115

0.2980

0.5463

-0.7104

-1.0192

0.7284Slide9

CPMs for Planning-Level AnalysisUrban Single-Lane Roundabouts

N =

exp

a

MAJAADT

b

MINAADT

c

exp

d

× NUMBERLEGS

Severity

a

b

c

d

k

TOT

-5.6049

0.3274

0.3960

-0.8681

0.5030

FI

-8.6597

0.5271

0.3505

-0.7317

0.3290

PDO

-5.5319

0.2653

0.4294

-0.9260

0.6064Slide10

CPMs for Planning-Level AnalysisUrban Two-Lane Roundabouts

N =

exp

a

MAJAADT

b

MINAADT

c

exp

d

× NUMBERLEGS

Severity

a

b

c

d

k

TOT

-5.6642

0.5210

0.2905

-0.4610

0.9263

FI

-10.3369

0.9134

0.1937

-0.5131

0.5611

PDO

-5.7669

0.4954

0.3098

-0.4618

1.0642Slide11

CPMs for Intersection-Level Design DecisionsCPMs used during design or retrofit work

Predict crashes per year for crashes within circulating roadway and on legs and considered related to roundabout geometry or operation

Pedestrian and bicycle crashes not included

CPMs developed for four site types

3 legs 1 circulating lane

3 legs 2 circulating lanes

4 legs 1 circulating lane

4 legs 2 circulating lanesSlide12

CPMs for Intersection-Level Design Decisions

Model form: N = N

SPF

× CMF

1

... ×CMF

2

N

SPF

=

exp

a+c

x

Irural

(ENTAADT/1000)bNSPF = predicted average crash frequency for base conditions on all legs, crashes/

yr

EntAADT

= entering AADT for roundabout,

veh

/d

I

rural

= area type indicator variable (= 1 if rural, 0 if urban)

CMFs developed including various geometric variables

SPFs developed for fatal+injury (FI) crashes and PDO crashes

Slide13

CPMs for Intersection-Level Design Decisions

Site/Severity

Type

a

b

c

k

FI

3 leg 1 circulating lane

-4.404

1.084

0.206

0.31

3 leg 2 circulating lanes

-3.887

1.306

0.250

0.36

4 leg 1 circulating lane

-3.503

0.915

0.206

0.33

4 leg 2 circulating lanes

-3.535

1.276

0.250

0.45

PDO

3 leg 1 circulating lane

-1.7200.4860.1680.543 leg 2 circulating lanes-1.5651.0550.4961.064 leg 1 circulating lane-1.4750.7020.1680.804 leg 2 circulating lanes-1.5361.1310.4960.79

N =

exp

a+c

x

Irural

(

ENTAADT/1000)

bSlide14

CPMs for Intersection-Level Design DecisionsCrash type distributions provided by site and severity type

Head-on

Right-angle

Rear-end

SSSD

Other – single-vehicle

Animal

Fixed-Object

Other-Object

Parked vehicle

Other – multiple-vehicleSlide15

CPMs for Intersection-Level Design DecisionsPedestrian and bicycle crashes too few to model so are not included

In data, 1% were vehicle-bicycle and 0.4% vehicle-pedestrianSlide16

CPMs for Intersection-Level Design DecisionsOptional application of Severity Distribution Functions

Break down estimates of FI into individual K, A, B, C estimates

Considers speed limits on approaches (between 10 to 60 mph)

Factor is leg specificSlide17

CPMs for Intersection-Level Design DecisionsSTEP 1

f

j,sl

=

exp

[3.1187 x {(

SL

j

/100)

2

– (35/100)

2

}]

f

j,sl = severity adjustment factor for leg j

SL

j

= speed limit on leg j in mphSlide18

CPMs for Intersection-Level Design Decisions

STEP 2

S

k

=

exp

[

F

k

] x [p

1

x f

1,sl

+ p

2 x f2,sl +p3

x f

3,sl

+p

4

x f

4,sl

]

S

A

=

exp[FA] x [p1 x f1,sl + p

2

x f

2,sl

+p3 x f3,sl +p4 x f4,sl ]SB = exp[FB] x [p1 x f1,sl + p2 x f2,sl +p3 x f3,sl +p4 x f4,sl ]Pi = AADTi/(AADT1+AADT2+AADT3+AADT4)P1to4 = proportion of total roundabout volume on legFK, FA and FB estimated for each combination of number of legs and circulating lanesSlide19

CPMs for Intersection-Level Design DecisionsSTEP 3

P

K

= S

K

/(1+S

K

+S

A

+S

B

)

P

A

= S

A/(1+SK+SA+SB

)

P

B

= S

B

/(1+S

K

+S

A

+S

B)PC = 1-(PK+PA+PB)Slide20

CMFs for Intersection-Level Design DecisionsTwo Types of CMFs Developed

Those that apply to entire roundabout

Those that apply to specific legs

Leg-level CMFs are combined into a entire roundabout CMF prior to applying to SPF

CMFs do not necessarily apply to all combinations of number of legs, circulating lanes and crash severities

Where CMFs are not provided the evidence was insufficientSlide21

CMFs for Intersection-Level Design DecisionsCMF

ICD

– Inscribed Circle Diameter

Applies to: urban roundabouts with a single circulating lane, ICD from 90 to 160 ft.

Severity: FI

CMF

ICD

=

exp

(-0.00621 x (ICD – 125)]Slide22

CMFs for Intersection-Level Design Decisions

CMF

outbd

– Outbound-Only Leg

Applies to: urban/suburban/rural that are a interchange ramp terminal with crossroad with only one outbound only leg

Severity: FI

1 Circulating Lane 0.426

2 Circulating Lanes 0.455Slide23

CMFs for Intersection-Level Design DecisionsCMF

bypass

– Right-Turn Bypass Lane

Applies to – Individual leg in any area type

Severity – FI

CMF – 0.355Slide24

CMFs for Intersection-Level Design Decisions

CMF

ap

– Access Point Frequency

Applies to – Individual leg in any area type

Severity – FI, PDO

CMF

ap

, FI

=

exp

(0.0659 x n

ap

]

CMF

ap

, PDO

=

exp

(0.0885 x n

ap

]

n

ap,

= number of driveways or unsignalized access points on leg within 250 ft of yield lineSlide25

CMFs for Intersection-Level Design Decisions

CMF

ew

– Entry Width

Applies to – Individual leg in any area type with 2 circulating lanes

Severity – FI, PDO

CMF

ew

, FI

=

exp

(-0.0300 x 29]

CMF

ew

, PDO

=

exp

(-0.0390 x 29]Slide26

CMFs for Intersection-Level Design Decisions

CMF

cl

– Circulating Lanes

Applies to – Individual leg in any area type (not outbound only legs)

Severity – FI, PDO

CMF

cl

, FI

=

exp

(0.1960 x (

n

cl

x n

el

-4)]

CMF

cl

, PDO

=

exp

(0.2190 x (

n

cl

x nel-4)]ncl = number of circulating lanesnel

= number of entering lanesSlide27

CMFs for Intersection-Level Design Decisions

Aggregating Leg-Level CMFs to apply to roundabout

Step 1

CMF

leg

=

CMF

bypass

x

CMF

ap

x

CMF

ew

x

CMFclStep 2

CMF

Rnbt

=(p

1

x CMF

1

)+(p

2

x CMF

2

)+(p3 x CMF3)+(p4 x CMF4)P

i

=

AADT

i/(AADT1+AADT2+AADT3+AADT4)P1to4 = proportion of total roundabout volume on legCMF1to4 = Combined CMFleg for each legSlide28

CPMs for Leg-Level Design DecisionsCPMs used during design or retrofit process

Predict crashes per year for crashes associated with a specific roundabout leg

Pedestrian and bicycle crashes not included

Crash Types Modeled

Entering-circulating (all severities and FI)

Exiting-circulating (all severities)

Rear-end (all severities)

Single-vehicle approach (all severities)

Single-vehicle approach + circulating (all severities)

Circulating-circulating (all severities)

Total (all severities)Slide29

CPMs for Leg-Level Design Decisions

Entering-Circulating Crashes

N =

exp

a

(ENTAADT/1000)

b

(CIRCAADT)

c

Exiting-Circulating Crashes

N =

exp

a

(EXTAADT/1000)

b

(CIRCAADT)

c

Rear-End Approach Crashes

N =

exp

a

(APPRAADT/1000)

b

(CIRCAADT)

c

SV Approach and SV Approach +SV Circulating Crashes

N =

exp

a

(CIRCAADT/1000)

bCirculating-Circulating CrashesTotal CrashesN = expa(APPRAADT/1000)b(CIRCAADT)cN = expa(APPRAADT/1000)bN = crashes per year related to legENTAADT – entering AADTEXTAADT – exiting AADTCIRCAADT – circulating AADTAPPRAADT – approach AADTSlide30

CPMs for Leg-Level Design Decisions

Severity

Entering

Lanes

Circulating Lanes

a

Coefficient

k

b

c

KABCO

1

1

-2.584

0.6091

0.3020

0.7470

1

2

-0.314

0.9636

0.3917

0.6232

2

1

-5.784

0.3608

0.6711

1.0734

2

2-3.0060.80540.73980.7759N = expa(ENTAADT/1000)b(CIRCAADT)cEntering-Circulating CrashesSlide31

CMFs for Leg-Level Design Decisions

CMFs developed for:

ICD – inscribed circle diameter

Angle – angle to next leg

Circwidth

– circulating width

NumberAccess

– number of access points within 250ft.

Luminaires – Number of luminaires within 250 ft.

PostedSpeed

– Posted speed limit on approachSlide32

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

change

Base

Value

minimum

maximum

Circulating Width

1

2

30 ft.

15 ft.

42 ft.

1.0324

2

1

25 ft.

45 ft.

0.9860

2

2

24 ft.

45 ft.

0.8715

ICD

1

1150 ft.65 ft.236 ft.0.993212110 ft.314 ft.0.991821135 ft.426 ft.0.9853Angle1290°53°182°0.97692269°182°0.9867Bypass Lane1

1

None

0.3685

Entering-Circulating Crashes – All SeveritiesSlide33

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

ICD

1

or 2

1

or 2

150 ft.

65

426

0.9951

Angle

1

or 2

1 or 2

90

°

37

°

186

°

0.9825Entering-Circulating Crashes – KABC SeveritiesSlide34

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

Circulating

Width

1

2

30

ft

15 ft.

42 ft.

1.198

2

1

30

ft

25 ft.

45 ft.

0.772

ICD

2

1

150 ft110 ft.426 ft.0.985Exiting-Circulating Crashes – All SeveritiesSlide35

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

NumberAccess

1

or 2

1

or 2

1

0

8

1.094

Luminaires

1

or 2

1 or 2

2

0

8

0.937

Rear-End Crashes – All SeveritiesSlide36

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

Posted Speed

1

or 2

1

or 2

40 mph

10 mph

60 mph

1.0451

Single-Vehicle Approach Crashes – All SeveritiesSlide37

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

Circulating

Width

1

or 2

1

or 2

30 ft.

24 ft.

45 ft.

0.917

Circulating-Circulating Crashes – All SeveritiesSlide38

CMFs for Leg-Level Design Decisions

Variable

Circulating Lanes

Entering Lanes

Variable Levels

CMF/unit

increase

Base

Value

minimum

maximum

PostedSpeed

1

or 2

1

or 2

40 mph

10 mph

60 mph

1.0356

CircWidth

1

or 2

1

or 2

30 ft.

14 ft.

45 ft.

0.9771

Single-Vehicle Approach + Single-Vehicle Circulating Crashes – All SeveritiesSlide39

Effect of a Driver Learning Curve

Testing thesis that driver behavior and safety may improve with familiarity

Developed intersection-level SPFs including a variable for years post construction

109 sites where opening data was known were used

Any trends are weak and vary from state to state

Based on findings there is no satisfactory evidence of a driver learning curve

Possible that DLC is for a shorter time period, e.g. over first few months of year 1, if one existsSlide40

ConclusionsCPMs developed for both intersection-level and leg-level design analyses for U.S. roundabouts

Estimate how geometric and operational features influence the number and severity of crashes

SPFs also developed for planning-level analyses or network screening

CPMs to be included in next edition of the HSM,

SafetyAnalyst

and IHSDM