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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
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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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.
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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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.
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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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
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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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)
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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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
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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[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]
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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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
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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Figure 9: PZ Probability of vehicle only in
blind zone
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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.
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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Figure 10: EL vs. qT and qL
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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
P e r c e n t i l e ( % )
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
Intersection Width = 35 m
Speed
(km/h)
3.5 m Lane 3.35 m Lane
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
Intersection Width = 40 m
Speed
(km/h)
3.5 m Lane 3.35 m Lane
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.
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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Figure 11: Available sight distance
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4 0 5 0 6 0 7 0 8 0 9 0 1 0
P e r c e n t i l e ( % )
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
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
13
Figure 13: EL by left turn phasing scenario (qL =
150 vph)
Figure 12: Improvement in sight distance
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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
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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Figure 14: Effect of detection loop positions loops
on average EL
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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
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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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.pdf8/3/2019 Mollett CARSP 2005 Left Turn
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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+
=
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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