Mollett CARSP 2005 Left Turn

8/3/2019 Mollett CARSP 2005 Left Turn

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Improving left turn safety at signalized

intersections

Calvin J. Mollett M(Eng)

Regional Municipality of York: Ontario

Abstract

The safety performance of left turn traffic atsignalized intersections depends on a complex

interaction of traffic volumes, intersection

geometry, operating speeds and signal phasing and

timing details. This paper develops and describes aquantitative model that can be used to assess the

potential impact of these factors on the safety andcapacity of left turn movements at signalizedintersections. In particular the model is used to

assess the effect of intersection design choices on

the potential safety and capacity of a left turnmovement. The paper shows that at intersections

designed according to provincial design standards in

Ontario (without fully protected left turn phasing),under certain conditions, do not have adequate left

turn sight distance, and that in such cases the safety

and capacity of a left turn movement not only

depends on the left turn volume and the opposingthrough volume (as assumed by the Highway

Capacity Manual) but also on the opposing left turn

volume and the adjacent through volume. Since acapacity guide such as the Highway Capacity

Manual and traffic analysis software such as

Synchro do not explicitly account for the effectsof intersection geometry and limited sight distances,

and may overestimate the capacity of a left turn

movement. Measures to improve left turn safety are

identified. These measures include providing

protected left turn phases, placing detector loopscloser to the stop line, decreasing the negative offset

between opposing left turn lanes and introducing ashadow lane between a left turn lane and the

adjacent through lane. Opportunities for

enhancements to existing intersection and signal

design guidelines are identified and

recommendations are made for further investigationand research.

Rsum

La sret du trafic qui vire gauche aux

intersections signalises dpend d'une interaction

complexe du volumes du trafic, la gomtrie delintersection, la vitesse de fonctionnement et mise

en phase des signaux et des dtails de

synchronisation. Cet article dveloppe et dcrit un

modle quantitatif qui peut tre employ pourvaluer l'impact potentiel de ces facteurs sur la

sret et la capacit de mouvements du trafic qui

vire gauche aux intersections signalises. En

particulier le modle est employ pour valuel'effet des choix de conception d'intersection sur la

sret et la capacit potentielle d'un mouvement devirage gauche. La recherche prouve qu'aux

intersections conu en accordance aux normes

provinciales de lOntario (sans virage gauch

entirement protg par une indication), danscertaines conditions, les intersections n'ont pas la

distance proportionne de vue pour un virage

gauche, et que dans ces cas-ci la sret et la capacitd'un mouvement de virage gauche dpend non

seulement du volume de virage gauche (commeassum par le manuel de capacit de route) maisgalement du volume d'opposition de virage

gauche et du volume traversant adjacent. Puisqu'un

guide de capacit tel que le manuel de capacit de

route et le logiciel d'analyse de trafic tel queSynchro nexpliquent pas explicitement les effets

de la gomtrie d'intersection et des distances

limites de vue, ils peuvent aussi surestimer lacapacit d'un mouvement de virage gauche. Des

mesures d'amlioration de la sret de virage

gauche sont identifies. Ces mesures incluenfournir des phases protges de virage gauche

plaant des boucles de dtection plus prs de l

ligne d'arrt, diminuant ainsi l'excentrage ngatifentre les ruelles opposante de virage gauche et

prsenter une ruelle d'ombre entre une ruelle d


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virage gauche et la ruelle traversante adjacente.Des occasions pour le perfectionnement des

directives existantes de conception d'intersection et

de signal sont identifies et des recommandationssont faites pour plus de recherche et de

dvelopment.

Introduction

To efficiently accommodate left turn vehicles at

signalized intersections a trade off is requiredbetween reducing delay and reducing collisions, not

only for left turn vehicles but for all vehicles using

an intersection.

The safety and operational performance of left turn

traffic at signalized intersections depends on a

complex interaction of traffic volumes, intersectiongeometry, operating speeds, signal phasing and

timing design.

For guidance on how to design intersections to

accommodate left turn vehicles traffic engineers

rely on intersection design guidelines such asTACs Manual for the Geometric Design

Standards for Canadian Roads or Provincial

guidelines such as MTOs Geometric Design

Standards for Ontario Roads.

This paper will assess how well the proposed

designs in these Guidelines perform with respect toproviding left turn sight distance to vehicles whose

sight lines are restricted by vehicles in the opposing

left turn lane, for a range of operating conditions.

A quantitative model will be developed to assess the

potential impact of designs, that provide inadequate

sight distance, on safety and capacity for differentsignal phasing and timing designs, and different

traffic volume combinations.

The model will be used to justify a hierarchy of

strategies to improve left turn safety.

Finally enhancements to current geometric designstandards for signalized intersections will be

recommended.

Proceedings of the Canadian Multidisciplinary Road Safety Conference XV; June 5-8, 2005; Fredericton, NBLe compte rendu de la XVe Confrence canadienne multidisciplinaire sur la scurit routire; 5-8 juin 2005; Fredericton, NB

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Problem Statement

Figure 1a and 1b illustrate a typical left turn conflict

scenario:

A driver wants to perform a left turn during

a permissive left turn phase, AND

The left turn sight distance is restricted by a

vehicle in the opposing left turn lane, AND

The available sight distance (SDa) is lessthan the sight distance required (SDr) for at

least the median lane of the opposing

approach, AND

Any one of the three scenarios in Figure 1bapplies to the through lanes of the opposing

approach.

It is assumed that a vehicle in the clear zone doesnot present a conflict situation as a rational driver is

unlikely to commence a turn while there is a clearly

visible vehicle in the clear zone.

A left turn conflict is assumed to be:

A collision between a permissive left turn

vehicle and an opposing through moving

vehicle

The sudden deceleration of a throughmoving vehicle to avoid a collision with a

permissive left turn vehicle

The sudden termination of a left turn

maneuver by a permissive left turn vehicle

to avoid a collision

The following measure of exposure to left turn

conflicts will be used to assess the potential effect

of traffic volumes, signal timing and phasing designand intersection geometry on left turn related

collisions:

C

PPLE ZSDuuL

)(3600 = [1]

C = Cycle length (sec)

Lu = Number of permissive left turns during a cycle

ru = Probability that the sight distance is restricted by an

opposing left turn vehicle during the permissive

phase

PSD = Probability that the restricted sight distance is less

than the required sight distance


3

SDr

Clear Zone

Blind Zone

SDa

Figure 1a: Illustration of left turn conflict

(a) (b) (c)

OR OR

Figure 1b: Illustration of left turn conflict scenarios


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Pz = Probability of either scenario (a) , (b) or (c) in

Figure 1b occurring when the restricted sight

distance is less that the required sight distance

EL is not a perfect measure of exposure. It has the

following shortcomings:

PSD and PZ will be determined using only the85th percentile approach speed and the 85th

percentile available sight distance

No allowances are made for trucks

The approach speed is treated as a constant

(at the 85th percentile level) and not as a variable

Vehicle arrivals are considered to be

completely random, thereby not allowing for the

effect of signal progression and platooning

It does not account for phase end left turn

sneakers during the intergreen interval

It does not account for drivers selectinginappropriate gaps under perfect sight distance

conditions

In spite of these shortcomings it is postulated that

there will be a strong positive correlation between

EL and the actual number of left turn relatedcollisions, and that any measure that is effective in

reducing EL will also be effective in reducing left

turn related collisions. However, due to the use of

an imperfect measure of exposure it cannot be

assumed that the relationship between EL andcollisions will be perfectly linear, and that a certain

% reduction in EL will translate to the same %reduction in collisions. It is therefore strongly

advised that EL should not be used as the

denominator in collision rate calculations.

Theoretical Framework

Equations 2 to 9 are based on procedures detailed in

the Highway Capacity Manual (2000).

Consider a signalized intersection with traffic

volumes shown in Figure 2. Both left-turn

movements are provided with a protected /permissive phase.

The effective duration of the protected left turn

phase for the study and opposing approaches are g

and g respectively. The duration of the permissive

left-turn phase for qL is equal to the effective greeninterval for the opposing through movement (gopp).

The Highway Capacity Manual (2000) providesprocedures for estimating gandgopp for approaches

with leading left turn phases.


4

Figure 2: Signal phasing, timing and traffic volume

parameters

C

Length

r g gq gu

Qa

Qg

Qu

Q

C

Length

r g gq

gu

Qa

Qg

Qu

Q

qL

qT

qL

T

OpposingApproach

qT

StudyApproach


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The period gopp consists of two components, gq and

gu.

When the permissive green phase is initiated the

opposing queue (qT) begins to move. While the

opposing queue clears, left turns are effectivelyblocked. The portion of permissive green blocked

by the clearance of an opposing queue is designated

gq. Once the opposing queue clears left-turn

vehicles filter through an unsaturated opposingflow. The portion of permissive green during which

left turns filter through the opposing flow is

designated gu.

For the study approach, the average queue at the

end of the red interval is:

3600/rqQ La = ... [2]

The average queue at the end of the protected left-

turn phase (g) is:

]3600/)(,0max[ gqsQQ Lpag = [3]

The average queue at the end ofgq is:

3600/qLgu gqQQ += ... [4]

The average queue at the end ofgu is:

]3600/)(,0max[ Lsuur qsgQQ = [5]

The duration of the effective red interval is:

oppggCr = [6]

qL = Left-turn flow (vph)

sp = Saturation flow rate for protected phase = 0.95ss = Saturation flow rate (1900 vph assumed)

g = Effective length of protected left-turn phase (sec)

ss = Maximum vehicle departure rate during gu (vph)

C = Cycle length (sec)

The value of ss can be determined by (HCM, 2000):

)3600/'exp(1

)3600/'exp()3600/'(

fT

cTT

stq

tqqs

= [7]

tc = Critical gap (4.1 sec assumed)

tf = Follow up time (2.2 sec assumed)

The value of gq can be determined as follows(HCM, 2000) :1:

L

T

Tq t

qs

grNqg

+=

'

))('/'( [8]

s = Saturation flow rate (1900 vph)qT= Opposing through volume (vph)

N = Number of opposing through lanestL = Lost time(4 sec assumed)

The Highway Capacity Manual (2000) providesadditional procedures to estimate gq for approacheswith leading left turn phases.

The value of gu can be determined by:

qoppu ggg = [9]

1 Assuming a platoon ratio and lane utilization factor = 1.


5


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Lu Number of permissive left turns

The number of permissive left turns during gu isgiven by:

],min[ uLusuu gqQsgL += [10]

500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700

q'T

-OpposingThroughVolume(vph)

0

1

2

3

4

5

6

7

8

9

Lug=12secandg'=12sec

g=0andg'=0

q'L

=250vph

Figure 3 illustrates the effect of introducingprotected left turn phases on the value of Lu. As qTincreases the value of gq increases (Equation 8),

hence the value of gu decreases (Equation 9),

leading to a reduction in Lu (Equation 10).

ru Probability that the left-turn sight distance

is restricted during gu

To estimate the probability that during the period guthe left turn sight distance is restricted by opposing

left turn vehicles, two different cases should be

considered.

Case 1 is illustrated in Figure 4 and Case 2 is

illustrated in Figure 5. In Case 1 gu > gu, and inCase 2 gu < gu.

In Figure 4 during the period gu1 left-turn vehiclesfilter through the opposing traffic stream, however

for the whole duration of gu1 the opposing left turnqueue is stationary. Therefore for Case 1 during guthe probability that the sight distance is restricted is

equal to 1, i.e. ru1 = 1. However in Case 2 no left

turns are possible during gu1 as the left turn vehicles

are still waiting for the opposing queue to clear. It istherefore of no consequence that the sight distance

may be restricted during this period. i.e. ru1 = 0.

In Figure 4, during the period gu2 the opposing left

turn queue clears as vehicles filter through available

gaps in the through traffic (qT). It is assumed thatwhile the opposing left turn queue clears the sight

distance will remain restricted for 100 % of the

time, i.e. ru2 = 1 . In Figure 5, by the time that left

turn vehicles have an opportunity to start turning

the opposing left turn queue has already been

clearing for a maximum period equal to gu1. For the


6

r g gq gu

r g gq gu

gu1

gu2

gu3

Qa

Qg

Qu

Qr

Qu

Qa

Qg

gu1

gu2

r g gq gu

r g gq gu

gu3

Qa

Qg

Qu

Qr

Qa

Qg

Qu

Figure 4: Case 1 gu < gu

Figure 5: Case 2 gu > gu

Figure 3: Relationship between Lu and qT, qL and g

Figure 4: Case 1 gu < gu


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remaining time to clear (if any) the probability thatthe sight distance is restricted is also equal to 1, i.e.

ru2 = 1.

Case 1: ]''

'3600,'min[2

Ls

uuu

qs

Qgg

= [11]

Case 2: ]''

'3600,0max[ 12 u

Ls

u

u gqs

Qg

=

[12]

During the period gu3 the probability of one or more

opposing left turn vehicles in a queue waiting forsuitable gap, and therefore restricting the sight

distance can be determined using queuing theory

(Taha, 1982) as follows:

]'

',1min[3

s

Lu

s

q= [13]

qL = Opposing left turn flow rate (vph)

ss = Maximum vehicle departure rate during gu (See

Equation 9)

The probability that the left-turn sight distance will be restricted during gu can be estimated by

estimating the weighted average of ru1, ru1 and ru1

over the period gu as follows:

u

uuuuuuu

g

ggg 332211

++= [14]

Figure 6 illustrates how the value of ru varies with

different values of qL and qT. It is evident that withvery high values of qT opposing left turn vehicles

are unable to find suitable gaps and restrict the left-

turn sight distance for the whole duration of gu, evenat low left turn volumes.

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

2300

2400

2500

2600

2700

2800

2900

3000

A p p r o a c h V o l u m e - qL

0 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

1 . 2

r

u

-Probabilityofrestrictedsightdistanceduringgu

2 5 0

2 0 0

1 5 0

1 0 0

5 0

O p p o s i n g L e f t T u r n V o l uL T

q L = 2 5 0

q 'T = 8 0 0

g = 0g ' = 0r = 5 4 s eC = 1 2 0

In reality, due to the unwillingness of drivers to

accept available gaps when their left turn sigh

distances are inadequate, the duration of gu2 will belonger and gu3 will be shorter than those calculated.

It has been observed that often opposing left turn

queues lock horns neither queue is moving

because the one queue restricts the left turn sighdistance of the other and vice versa, even when

there are adequate gaps in the opposing traffic

streams. In these cases gu3 would be equal to zero

resulting in ru =1.

PSD - Probability that restricted sight distance is

less than the required sight distance.

SDr- Sight Distance Required

Harwood et al. (1996) suggested that at locationswhere left turns from the major road are permitted

at signalized intersections without a protected turn

phase, sight distance along the major road should beprovided based on a critical gap approach.

vGSD r 278.0= [15]

SDr= Required sight distance for left turn from the major

road (m)

v = 85th percentile speed on major road (km/h)


7

Figure 6: Relationship between ru and qL and qT


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G= Critical gap size for left-turn from the major road

(sec)

Table 1 shows the G values recommended byHarwood et al. (1996).

Table 1: Critical Gap values

Vehicle Type Number of opposing through lanes

1 Lane 2 Lanes 3 lanes

Passenger Cars 5.5 sec 6.0 sec 6.5 sec

Single-unit trucks 6.5 sec 7.2 sec 7.9 sec

Combination trucks 7.5 sec 8.2 sec 8.9 sec

These recommended gap sizes will provide enoughtime for a left turn driver to decide on a course of

action, and to perform a left turn movement without

impeding opposing through moving vehicles.

SDa - Available Sight Distance

McCoy et al. (1999) devised a procedure to

calculate the available sight distance for any

intersection based on its geometry. They defineAvailable Sight Distance (SDa) as the distance from

the left-turn drivers eye to the point at which

his/her line of sight intersects the centreline of thenear opposing through lane.

During their study McCoy et al. (1999) studied,

using video digital technology, the position of morethan 2,500 vehicles at 6 intersections. Regression

analysis of the vehicle positioning data was used to

determine the relationship between available sightdistance and various intersection design parameters

as illustrated in Figure 5. Analyses were conducted

for the 95th, 85th, 75th, 65th and 50th percentile vehiclepositions in order to develop guidelines for a range

of sight distance design levels and intersection

design parameters.

biaai YYSD += [16]

ppopLLp

OLTLpopLLpwia

bikWkxkWk

WkxkWkVYYY

LtL 8765

321)((

+++

++++=

Ya = Distance between opposing vehicles

Vw = Width of design vehicle (2.15m assumed)

Yi = Longitudinal distance from the front of the left-turn

vehicle to the drivers eye. (1.5m assumed)p = Percentile value

WLL = Width of left-turn lane line (shadow lane)WOTLT = Width of opposing left-turn lane

WLTL = Width of left-turn lane

kip = Constant i for p-percentile vehicle position

x0 = Negative offset between opposing left turn lanes

W = 0.5Lw to determine SDa1 and = 1.5Lw to determine

SDa2

For intersection approaches with a raised median it

is desirable that the sum of WLL and -x0 be equal to

the width of the median.

Table 2: Constant values for sight distance equation

Constan

t

Percentile Position

50 65 75 85 95

k1p -0.58 -0.53 -0.50 -0.48 -0.45

k2p -0.31 -0.28 -0.24 -0.21 -0.20

k3p 0.40 0.39 0.34 0.36 0.25

k4p 4.74 4.52 4.75 4.28 4.97

k5p -0.97 -1.02 -1.02 -1.05 -1.07

k6p -0.30 0.36 -0.41 -0.46 -0.54

k7p 0.35 0.36 0.40 0.46 0.41

k8p -2.62 -2.17 -2.39 -2.66 -1.97


8

[1


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Equation 17 and the parameters in Table 2 were

derived assuming that left turn vehicles do not enter

into the intersection to wait for an available gap, asis the common practice. This practice however does

not necessarily improve the available sight distance

for left turn vehicles. The benefits of a smaller

offset (xo) could be decreased or eliminated by theeffect of a smaller distance between opposing

vehicles (Ya).

The available sight distance percentile values (SDai)

can be estimated by substituting the corresponding

percentile constants in Table 2 into Equations 16

and 17.

Figure 8 shows the available sight distance

percentiles for a typical signalized intersection of 2-lane arterials in York Region, designed according to

MTOs Geometric Design Standards for Ontario

Roads and TACs Manual of Geometric Design

Standards for Canadian Roads, i.e. offset = -2 m

intersection width = 35 m, left turn lane widths = 3

m, through lanes widths = 3.5 m and shadow lanewidth = 0.

4 0 5 0 6 0 7 0 8 0 9 0 1 0

P e r c e n t i l e ( % )

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

1 6 0

SDa

-AvailableSightDistance(m)

The probability that the required sight distance isless than required (PSD) can be determined from

Figure 8 and Equation 18.

100/1 PercentilePSD = . [18]

For example, for a speed of 80 km/h the required

sight distance = 133 m (from Equation 15). From

Figure 8 the corresponding percentile value isapproximately 56 %. Therefore PSD = 0.44.

PZ - Probability of vehicles in blind zones

The procedure below estimates the probability of

either scenario (a), (b) or (c) in Figure 1b occurring,

and assumes a displaced exponential distribution inthe headways between vehicles during the period gu(as recommended by Troutbeck and Brilon).

)(2)2( 21 mr

maa t

v

SDt

v

SD

v

SD

Z eeP+

=

[19]


9

Ya

LW

WLTL

WLL

xo

WOLTL

Figure 7: Intersection design parameters

Figure 8: Available sight distance percentiles


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)'3600/'(1

)'3600/'(

Nqt

Nq

Tm

T

= [20]

tm = Minimum head way = 3600/1900 = 1.89 sec.

SDa1 = 85th Percentile available sight distance to Lane 1

SDa2 = 85th Percentile available sight distance to Lane 2

SDr= Sight distance required

v = 85th percentile speed on major road (km/h) N = Number of opposing through lanes

Figure 9 illustrates the non-linear relationship

between PZ and qT for a number of operating

speeds.

1 0 03 0 05 0 07 0 09 0 01 1 0 01 3 0 01 5 0 01 7 0 01 9 0 02 1 0 02 3 0 02 5 0 02 7 0

q 'T - O p p o s s i n g T h r o u g h V o l u

0 . 0 0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0 . 1 0

0 . 1 2

0 . 1 4

PZ

v = 9 0 k m / h

v = 8 0 k m / h

v = 7 0 k m / h

Left turn capacity

The capacity of a movement can be defined as the

maximum number of vehicles per hour that can

perform that movement given the intersectionstraffic flows and design.

Assuming a maximum of two sneakers per cycle (as

recommended by HCM, 2000), the maximumnumber of vehicles that can turn left during a cycle

is given by:

2)()( ++= supL sgsgC [21]

Equation 9, which is used to calculate s s assumesthat all available gaps are accepted by turning

vehicles. In reality drivers do not accept al

available gaps as they are not willing to putthemselves in danger when their sight distances are

restricted by opposing left-turn vehicles. Equation

32 is therefore likely to overestimate the truecapacity of a left turn movement.

Capacity estimation procedures in the Highway

Capacity Manual (2000), and the Canadian CapacityGuide for Signalized Intersections (1995) do no

explicitly consider the potential effect of an

intersections geometry (primarily intersectionwidth, offset and shadow lane width) and the traffic

flows qL and qT on a left turn movements capacity.

Certain warrants for protected left turn phasing rely

on the volume to capacity ratio. For exampleaccording to MTOs Guidelines for Traffic

Control Signal Timing and Capacity Analysis at

Signalized Intersections (1989) a protected lef

turn phase is warranted if the v/c ratio for a left turn

movement is larger than 1. Overestimation of thecapacity will lead to smaller v/c ratios which could

result in protected left turn phases not being

implemented where they do have the potential toefficiently improve traffic safety and operations.

Equation 22 presents a modified capacity equationthat takes into consideration the effect of left turn

queues and intersection geometry.

)1()((mod) ++= SDuussTuuSDpL PgssRgPsgC

[22]

RT= Probability that driver will perform a turn when PSD> 0.

RT is related to a drivers willingness to accep

uncertain gaps and is likely a function of:

A drivers perception that the next gap in

traffic is larger than the critical gap (G)required to perform a safe left turn.

The size of the blind zone

The time already spend waiting to turn left


10

Figure 9: PZ Probability of vehicle only in

blind zone


11/16

The potential reduction in capacity as a result ofrestricted sight distances is given by:

)1( = TSDusu RPsgC [23]

To calculate the minimum value of CL(mod) and amaximum value forC, RT = 0 should be assumed.

Example

Consider a typical signalized intersection:

Designed according to TAC and MTO

standards, with the following geometry: Offset

= -2 m, intersection width = 35 m, left turn

lane widths = 3 m, through lanes widths = 3.5

m and shadow lane width = 0.

With traffic volumes: qL = 250 vph and qT =

800 vph.

With signal timing parameters: g = 0, g = 0, r= 54 sec and r= 54 sec and C = 120 sec.

For this intersection Figure 10 illustrates thatbecause PSD and PZ > 0 the safety (as measured by

EL) of a left turn movement not only depends on the

opposing through volume (qT) but also the adjacentthrough volume (qT) and the opposing left turnvolume (qL).

5 0 0 7 0 0 9 0 0 1 1 0 0 1 3 0 0 1 5 0 01 7 0 0 1 9 0 0 2 1 0 0 2 3 0 0 2 5 0 0 2 7 0 02 9 0 0

q T - A p p r o a c h V o l u m e ( v p h )

0

2

4

6

8

1 0

1 2

1 4

1 6

EL

2 5 0

2 0 0

1 5 0

1 0 0

5 0q ' L - O p p o s i n g l e f t - t u r n v o l

P Z = 0 . 1 2 3

P S D = 0 . 4 4

q L = 2 5 0 v p h

q 'T = 8 0 0 v p h

If EL = 13.5, then from Equation 34 the maximum

reduction in capacity is 170 vehicles per hour

(assuming RT =0), which is about 58 % of the HCMcapacity (CL) of 293 vehicles per hour.

Improvement Strategies

Left turn safety improvement strategies should aim

to reduce the values of Ls, ru, PSD and PZ.

As long as there are left turn vehicles using an

intersection Ls and ru can never be zero, unless

permissive left turn movements are prohibite

completely through the introduction of exclusiveleft turn phases.

Through appropriate intersection design the valuesof PSD and PZ can be reduced to zero. Therefore the

greatest improvement in safety will likely be

attained by making intersection improvements to

reduce PSD and PZ. Should this not be feasiblemeasures to reduce Ls and ru should be considered

A first consideration should be to provide exclusive

protected left turn phases. Should this not b

feasible, due to increases in delay and othercollision types, providing a protected phase that is

called by appropriately located detection loops

should be considered.

Improving intersection sight distance

A combination of two strategies can be employed to

reduce PSD and PZ:

a) Reduce the sight distance required (SDr) byreducing the approach speed.

b) Increase the available sight distance (SDa)

by reducing the offset between opposing leftturn lanes (xo) and increasing the width of

the shadow lane (wLL).

Strategy (a) i.e. reducing approach speeds is

difficult to achieve and to sustain.


11

Figure 10: EL vs. qT and qL


12/16

Figure 11 illustrates the dramatic improvements in

sight distance that can be achieved with Strategy (b)

i.e. introducing shadow lanes and reducing thenegative offset between opposing left turn lanes.

4 0 5 0 6 0 7 0 8 0 9 0 1 0


0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

SDa-AvailableSightDistance(m)

W L L = 0 . 7 5 m a n d x0 = 1 . 2 5 m

W L L = 0 m a n d x0 = 2 m

W L L = 0 . 2 5 m a n d x0 = 1 . 7 5 m

W L L = 0 . 5 0 m a n d x0 = 1 . 5 0 m

The total amount that an intersection has to bewidened by (compared to the standard design) to

achieve these sight distance improvements is equal

to WLL.

Table 3 provides values for WLL and x0 for variety

of approach speeds, intersection widths and lane

widths that will ensure that the 85th

percentileavailable sight distance is equal or greater than the

required sight distance. i.e. PSD = 0.15.

It is evident that the current design standard of WLL= 0 and x0 = -2 m only provides adequate sight

distance (at the 85th percentile level) at lowapproach speeds and very wide intersections. As

speeds increase and intersection widths become

narrower the need for wider shadow lanes andsmaller negative offsets becomes more urgent.

Table 3: Recommended Intersection Design Parameters

Intersection Width = 30 m

Speed(km/h)

3.5 m Lane 3.35 m Lane

WLL Offset WLL Offset

40 0 -2 0 -2

50 0.20 -1.8 0.20 -1.8

60 0.45 -1.55 0.45 -1.55

70 0.55 -1.45 0.55 -1.45

80 0.65 -1.35 0.65 -1.35

90 0.70 -1.3 0.70 -1.30


Speed

(km/h)


WLL Offset WLL Offset40 0 -2 0 -2

50 0 -2 0 -2

60 0.2 -1.8 0.25 -1.75

70 0.4 -1.6 0.45 -1.55

80 0.5 -1.5 0.55 -1.45

90 0.6 -1.4 0.60 -1.40


Speed

(km/h)


WLL Offset WLL Offset

40 0 -2 0 -2

50 0 -2 0 -2

60 0 -2 0 -270 0.25 -1.75 0.30 -1.70

80 0.40 -1.6 0.45 -1.55

90 0.50 -1.5 0.55 -1.45

At existing intersections it may not always be

possible, nor feasible, to reduce the negative offset

as this may require extensive intersection re-construction work. It is however possible to

introduce a shadow lane by reducing the widths of

the through lanes. In York Region the standard lanewidth on arterials is 3.5 m. Should this be reduced

3.35 m each, on a 2 lane roadway, a shadow lane

0.3 m wide is possible. Figure 12 illustrates theimprovement in sight distance that this

improvement could achieve.


12

Figure 11: Available sight distance


13/16

4 0 5 0 6 0 7 0 8 0 9 0 1 0


6 0

8 0

1 0 0

1 2 0

1 4 0

1 6 0

1 8 0

2 0 0

SDa

-Availab

leSightDistance(m)

W L L = 0 . 3 a n d x0 = 2 m

W L L = 0 a n d x0 = 2 m

Y a = 3 5 m

v = 8 0 k m

S Dr = 1 3 3 m

It is evident from Figure 12 that the value of PSDwill decrease from 0.44 to 0.34.

Protected/Permissive Left Turn Phasing

Protected phases are called by detection loops inthe left turn lane. In York Region it is the practice to

place the detection loops at the 3rd vehicle position

in the left-turn lane. As a result the protected left-turn phases are only called when there are 3 or

more vehicles in the queue after the red interval.

There are four possible left turn phasing scenariosas shown in Table 4.

Table 4: Possible left turn phasing scenarios

Scenario Condition

g = 0 and g = 0 Qa < x AND Qa < x

g > 0 and g = 0 Qa >=x AND Qa < x

g > 0 and g > 0 Qa >=x AND Qa >= x

g = 0 and g = 0 Qa < x AND Qa >= x

x = Loop position in approach left turn lane (veh) x = Loop position in opposing left turn lane (veh)

Figure 13 illustrates the reduction in EL that can beachieved by introducing a protected left turn phase.

5 0 0 7 0 0 9 0 0 1 1 0 0 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 1 0 0 2 3 0 0 2 5 0 0 2 7 0 0 2 9 0 0

q T - A p p r o a c h V o l u m e ( v p h )

0

2

4

6

8

1 0

1 2

1 4

1 6

EL

g = 0 a n d g ' = 0

g = 0 a n d g ' = 1 2 s e c

g = 1 2 s e c a n d g ' = 0

g = 1 2 s e c a n d g ' = 1 2 s e

The actual value for EL for the design hour will be acombination of the EL values for each scenario

depending on the probability of each scenariooccurring. The probability of a scenario appearing

during a cycle depends on the duration of the red

interval and the left turn flow rate, and the positionof the detection loops.

To assess the impact of detection loop positions the

following procedure was adopted to calculate EL:

= =

=sr

i

sr

j

jiLjiL EppE0 0

),(' [24]

!

)()(

i

rqep

i

L

rq

i

L

= [25]

!

)'('

)'(

j

rqep

j

L

rq

j

L

= [26]

If i >= x then EL and EL is estimated assuming g = 12 sec.

If j >= x then EL and EL is estimated assuming g = 12 sec

i = Number of vehicles in left turn queue after red

interval

j = Number of vehicles in opposing left turn queue afterred interval


13

Figure 13: EL by left turn phasing scenario (qL =

150 vph)

Figure 12: Improvement in sight distance


14/16

pi = Probability of i vehicles in left turn queue after red

interval

pj = Probability of j vehicles in opposing left turn queue

after red interval

EL(i,j) = Exposure to left turn conflicts on study approachwhere Qa = i and Qa = j

x = Loop position left turn lane

x = Loop position in the opposing left turn lane

Figure 14 illustrates the effect of moving the

detection loops closer to the left turn lane stop line.

5 0 0 7 0 0 9 0 0 1 1 0 0 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 1 0 0 2 3 0 0 2 5 0 0 2 7 0 0 2 9 0 0

qT

- A p p r o a c h v o l u m e ( v p h )

0

2

4

6

8

10

12

14

16

EL

N o L o o p s

x = 3 a n d x ' = 3

x = 2 a n d x ' = 2

x = 1 a n d x ' = 1

qL

= 2 5 0 v p h

q'T

= 8 0 0 v p h

q'L

= 1 5 0 v p h

At qT = 1000 vph, changing from a permissive only

signal design (no loops) to a protected/permissivedesign with detection loops at the 3rd vehicle

position reduces EL by an average of 63 %.

Compared to placing the loops at the 3rd vehicle position, placing the loops at the stop line (1st

vehicle position) reduces the EL by 17 %.

In this case a reasonable compromise between

safety and delay may be achieved if the loops are

placed at the 2nd vehicle position instead.In most cases signal timing and phasing measures to

improve left turn safety will have the negative side

effect of reducing delay to through moving vehicles,

which in turn could cause other collision types toincrease. However, any need to change signal

timing and phasing parameters to improve safety, at

the expense of overall delay and congestion, can beavoided by taking measures to reduce PSD and PZinstead.

Conclusions

At intersections where PSD > 0 the capacity

and safety of left turn movements not only

depend on the left turn volume (qL) and theopposing through volume (qT) but also on the

opposing left turn volume (qL) and approachthrough volume (qT).

Current TAC and provincial design standardsin Ontario, for signalized intersections, do not

provide adequate left turn sight distance (a

the 85th percentile level) for all approach speedconditions when there is a vehicle in the

opposing left turn lane and exclusive left turn

phases are not provided

It is likely that the Highway Capacity Manual

and traffic analysis software such as Synchro

overestimate the capacity of a left turnmovement as they do not account for drivers

not accepting all available gaps as a result of

their sight distances being restricted byopposing left turn vehicles.

Significant reductions in EL can be achieved by decreasing the negative offset between

opposing left-turn lanes and/or increasing the

widths of shadow lanes.

The introduction of protected left turn phases

even when not warranted according to the

current guidelines, can have a significaneffect in reducing the exposure to left turn

collision conflicts.

The position of the detection loops in the left-

turn lane influences the likelihood tha

protected phases will be called and thereforehas an effect on the value of EL. It appears tha

there may be some justification to place the

detection loops at the 2nd vehicle positionrather than the 3rd vehicle position.

Recommendations

Consider incorporating the information in

Table 3 into intersection design guidelinessuch as TACs Manual of Geometric Design

Standards for Canadian Roads and MTOs


14

Figure 14: Effect of detection loop positions loops

on average EL


15/16

Geometric Design Standards for Ontario

Roads.

As a general design principle intersectionsshould be designed such that at least the 85th

percentile available sight distance (when

restricted) exceeds the required sight distance.

At intersections where the 85th percentile

available sight distance is less than therequired sight distance, the potential impact of

signal timing and phasing design decisions on

safety should be considered and evaluated

explicitly.

Decisions on whether or not to implement

protected left turn phasing should not rely on

theoretical capacity and delay estimationsmade by procedures in the Highway Capacity

Manual and traffic analysis software such asSynchro but rather on actual field

observations.

Future Research

Future research could aim to improve the usefulness

of the model presented in this paper by:

By establishing, through regression analysis,the relationship between EL and left turn

collisions.

Incorporating procedures to estimate changes

in delay to left turn traffic and other traffic as

a result of changes in left turn phasing, timingand intersection geometry.

Incorporating procedures to perform benefit-

cost analyses towards achieving an optimaltradeoff between safety and delay.

Acknowledgements

I wish to acknowledge the support of the RegionalMunicipality of York and in particular the support

of my supervisors Zoran Postic and Brian Harrison

as well as my colleagues, Mike Horne, DuaneCarson and Nelson Costa.

References

Harwood, D.W., Mason, J.M., Brydia, R.E.

Pietruchia, M.T. and Gittings G.L. ; NCHRP Report383: Intersection Sight Distance; TRB, Nationa

Research Council, Washington D.C, 1996.

McCoy, P.T., Byrd, P.S. and Pesti G; Pavement

Markings to Improve Opposing Left-Turn Sigh

Distance; Mid-America Transportation CentreLincoln, Nebraska; 1999.

Ministry of Transportation, Ontario (MTO)Geometric Design Standards for Ontario Highways

1985.

Ministry of Transportation, Ontario (MTO); TrafficSignal Timing and Capacity Analysis at Signalized

Intersections; 1989.

Institute of Transportation Engineers; Canadian

Capacity Guide for Signalized Intersections; 2n

edition; 1995.

Taha, H.A; Operations Research An Introduction

Macmillan Publishing Co. Inc,; New York; 1982.

Transportation Association of Canada; Manual of

Geometric Design Standards for Canadian Roads

1986Transportation Research Board; Highway Capacity

Manual; National Research Council; Washington

D.C.; 2000.

Troutbeck R.J., Brilon, W,; Unsignalized

Intersection Theory;

http://www.tongji.edu.cn/~yangdy/TrafficFlow/chap8.pdf


15
http://www.tongji.edu.cn/~yangdy/TrafficFlow/chap8.pdfhttp://www.tongji.edu.cn/~yangdy/TrafficFlow/chap8.pdfhttp://www.tongji.edu.cn/~yangdy/TrafficFlow/chap8.pdfhttp://www.tongji.edu.cn/~yangdy/TrafficFlow/chap8.pdf


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Appendix A

With reference to Figure 1b, the probability ofScenario (a), (b) and (c) occurring are:

]][[)()()( 1 mrmrma tttt

tt

Za eeeP

=

]][[)()()( 2 mrmrma tttt

tt

Zb eeeP

=

]][[)()()()( 21 mrmamrma tttttt

tt

Zc eeeeP

=

If ta2 >= tr then ta2 = tr

vSDt aa /11 = and vSDt aa /22 = and vSDt rr /=

The probability of Scenario (a) or (b) or (c)

occurring is given by:

ZcZbZaZ PPPP ++=

Substituting into this equation the equations for PZa,

PZb and PZa the following equation can be derived:

)(2)2(

21

m

r

m

aa

tv

SD

tv

SD

v

SD

Z eeP+

=


16

Documents

Mollett CARSP 2005 Left Turn