A STUDY ON THE SATURATION FLOW AND ITS INFLUENCING FACTORS
FOR SIGNALIZED INTERSECTIONS UNDER HETEROGENEOUS TRAFFIC
CONDITIONS.
Savitha B G1, R Satya Murthy2, H S Jagadeesh3, H S Sathish4, T SundarRajan5
1Assistant
Professor, Department of Civil Engineering, KSSEM, Bangalore,560062, India, [email protected]
Professor, Dept. of Civil Engineering, BMS Engineering College, Bangalore, 3Professor, Department of Civil
Engineering,BMSCE,Bangalore,560019,India , [email protected] ,4 Associate Professor, Department of Civil
Engineering,BMSCE,Bangalore,560019,India, [email protected] , 5Professor Department of Civil Engineering,
PEC,Puducherry605014,India, [email protected],
2
Abstract:
The intersections on urban roads in India generally cater to heterogeneous motorised traffic, along with slow-moving
traffic including pedestrians. It is therefore necessary to consider saturation flow for mixed traffic conditions to
evaluate the overall operation of signalized intersections. A proper traffic model must consider the varying
characteristics of all the road users to effectively design and efficiently manage the signalized intersections. This paper
presents the results of the study on analyses of saturation flow rate conducted at signalized intersections with mixed
traffic condition in the city of Bangalore, India. Studies were carried out at 30 signalized intersections at Bangalore
with varying conditions. The various adjustment factors affecting the saturation flow were found to be width of the
road, percentage of right and left turning movements, gradient, composition of the traffic and right turning radius. In
this paper ,30 signalized junctions in Bangalore urban area, with adjustment factors for right turn and left turn were
studied. By the introduction of the adjustment factors to the saturation flow rate can give better picture of the field
conditions, especially under heterogeneous traffic conditions of an urban area.
1
Introduction:
At-grade intersections are one of the most critical elements that influence the performance of an urban road network.
For a safe and efficient movement of large volumes of traffic of a city road network, majority of the intersections are
usually signalized. The most significant parameter that influences the design of a signalised intersection and its signal
plan is the “saturation flow”. Saturation flow is a key factor determining the capacity and level of service (LOS) of a
signalized intersection. It is revealed that traditional methods which are mainly developed using the average value of
observed queue discharge headways to estimate the saturation headway, might lead to the underestimate of saturation
flow rate. Operation and performance of signalized intersections is influenced by the roadway parameters, traffic
condition, operating parameters and environmental conditions, along with user’s behavioural characteristics which
significantly differ among locations.
Signalized intersections use a common form of traffic control to address roadway operations. Signalized intersections
allow road users to access new streets and change of direction of travel. Intersections should be able to serve their
varying traffic demands, provide minimum delay in passage, and maximum safety to all types of users especially
pedestrians. One evaluates the functioning of a typical signalized intersection in terms of two parameters: (1) capacity,
i.e., volume to-capacity (v/c) ratio, and (2) the level of service (LOS), with its delay and queue ranges. These
parameters are functions of traffic volume characteristics, signal characteristics, and geometry of the intersection. One
evaluates the capacity on the concept of saturation flow, whereas level of service(LOS) is evaluated based on the delay
that a user experiences, while crossing an intersection.
2
Literature Review:
Several researches have been conducted on HCM signalised intersection model, its applicability, and modifications.
This section presents research studies addressing some of the important aspects of traffic models applicable to
modelling heterogeneous traffic movement at signalized intersections. Researchers have developed various models to
evaluate the effectiveness of signalized intersections in terms of their capacity and level of service. The capacity will
be related to saturation flow rate at signalised intersections and LoS is defined based on delay at signalised intersection,
function of degree of saturation flow rate. Some of the research works related to saturation flow rate for signalised
intersections have been presented in this section.
2.1
Highway capacity manuals.
The Highway Capacity Manual (HCM) (1)is published by the Transportation Research Board(TRB) of the National
Academies of Science in the United States. It gives concepts, guidelines, and computational procedures
for calculating the capacity and quality of service of various highway facilities, including freeways, highways, arterial
roads, roundabouts, signalized and un-signalized intersections, rural highways, and the effects of mass
transit, pedestrians, and bicycles on the performance of these systems. There have been five editions with improved
and updated procedures from 1950 to 2010, and two major updates to the HCM 1985 edition, in 1994 and 1997.The
HCM has been a worldwide reference for transportation and traffic engineering scholars and practitioners, and also
the base for several countries- specific capacity manuals.
In India, the Central Road Research Institute (CRRI) one of the research institute under the umbrella organisation
Council of Scientific and Industrial Research (CSIR), has undertaken a national study to develop the Indian Highway
Capacity Manual (Indo-HCM) (2)
2.2
Study of saturation flow rate and its influencing factors in Indian context:
While IRC:106-1990 (3) -Guidelines of capacity of urban roads in Plain areas, gives an insight into the capacity of
urban roads in plain areas, it does not mention about saturation flow rate and LoS for signalised intersections.
IRC SP:41-19 (4) 94-Guidelines for the design of at grade intersections for rural and urban area, has defined saturation
flow as S=525* W PCU/hg (S =Saturation flow & W-Width of road in m) for roads having width above 5.5 m and
gives saturation flow rate based on radius of right turning vehicles. However, it does not specify anything about
defining LoS for signalised intersections.
Several researches have been carried out to study the saturation flow and its influencing factors relevant to Indian
traffic conditions. One of the studies (5) on the ongoing development of Highway Capacity Manuals in Indonesia,
Malaysia and China have shown that the traffic performance of traffic facilities in densely populated Asian countries
cannot adequately be predicted using western capacity manuals such as developed in the U.S, Australia and elsewhere.
Based on one of the study (6) it is possible to compare lane-based homogeneous traffic and non-lane-based
heterogeneous traffic. The above study was more on the methodology for obtaining dynamic PCEs and saturation flow
model, than on the PCU values and the model themselves. Another study (7) conducted at USA found that external
factors affecting the saturation flow rate are site specific. In another study (8) Free flow speed (FFS) and average travel
speed during peak and off- peak hour inventory of road segments is used for different urban street classes and speed
ranges of LOS categories were determined and it is found to be lower than that mentioned in HCM-2000.Based on
the study to understand the effect of two wheelers on saturation flow (9) it was studied and found that saturation flow
decreases with increase of two wheeler composition. Results from another research (10) confirm that the methodology
for saturation flow rates, put forward by highway capacity manual (HCM) can also be used in India. However,
parameters should be systematically calibrated, based upon widespread study, before they can be used effectively in
the practice of traffic control in India. Studies conducted in Malaysia (11) shows that the right-turn adjustment factor
decreases for higher proportions of turning vehicles and lower turning radius. This studies suggests that the US HCM
1994 equation does not account for Malaysian traffic conditions.
Given the above background and importance, this present study gives an insight in to the traffic flow parameters
influencing the saturation flow at signalised intersections of Bangalore urban area. This study mainly focusses on the
influencing factors such as width of the road and turning movements such as right turning and left turning movements
at the signalised intersection.
3
Objective
The study has following major objectives: (1) evaluation of the applicability of the HCM 2000 and Indonesian HCM
model to prevailing Indian conditions by determining the saturation flow at signalized intersections in India, and (2)
development of adjustment factors that account for the proposed modifications and deal with applicability and
establishing the need for modification there of issues of using the HCM signalized intersection model under Indian
urban conditions.
4
Site Selection
30 signalized intersections from Central Business District (CBD) of Bangalore area were selected for the study. It has
been the experience of the people of Bangalore over the past several years that all the intersections are facing a heavy
traffic congestion during peak hours. 11 -3 legged junctions and 19-4 legged junctions were considered for the study.
5
Data Collection:
During primary survey, data of land use, traffic, topographical and environmental features of the study area were
collected. The data was collected by videography method. The Traffic Management Centre (TMC) (12) at Bangalore
city, is the hub of a transportation management system, where information about the transportation network shall be
collected and combined with other operational and control data to manage the transportation network and to produce
traveller information. Videos were obtained from TMC, Infantry road, Bangalore. And supplementing them by site
visits, the following data were obtained: (1) through flows, right-turn flows, and left turn of traffic entities, (2)
geometrical characteristics of the intersection and (3) The signal cycle timings and volume count at each signalised
intersection were also collected. Representation of the signal data, geometric data and traffic data for a typical junction
is shown in Figure (1), Figure (2) and Figure (3) respectively.
Figure 1: Signal data of typical junction
Figure 2 : Geometric data of typical junction
Figure 3: Traffic data from videography for typical junction.
6
Measurement of Saturation Flow:
There are three principle methods available for the calculation of saturation flows:
The Road Note 34 Method (1963) (13)
The procedure used in this method consists of taking classified counts of vehicles crossing the stop line, within the
approach width, in six second intervals during the green and amber period of the cycle, under saturated flow condition.
An average number of 30 cycles is recommended to be used for each approach.
The Average Headway Method
This is the most commonly used alternative to the counting method. This method requires data on time headway
between vehicles as they cross the stop line. Time headway of a vehicle is measured as the time between the crossing
of the stop line by the rear bumper of the vehicle preceding it, and its own rear bumper.
Multiple Liner Regression Methods
In recent years a number of alternative methods of processing the data collected in classified vehicle counts format
have been developed, in an attempt to obtain simultaneous estimations of all properties of the discharge process. These
methods involved a multiple linear regression technique, which has been used by a number of researchers
7
Basic Model
The amount of traffic that can pass through a signal controlled intersection from a given approach depends on the
green time available to the traffic and on the maximum flow of vehicles pass the stop line during the green period.
When the signal changes to green, vehicles take some second to start and accelerate to normal speed. After a few
second the queue discharges at constant rate called the saturation flow (S). The saturation flow is the flow, which
would be obtained if there was a continuous queue of vehicles and they were passed at green time, or the saturation
flow is the maximum departure rate, which can be achieved when there is a queue. The saturation flow is generally
expressed in vehicles per hour green time. Figure 4,it could be seen that the average rate of flow is lower during few
minutes because vehicles are accelerating to normal running speed.
Figure 4 : Saturation flow rate, Source HCM 2000
Wide variation in the observed saturation flow resulted in the development of models for predicting saturation flow,
Webster and Cobbe (1966) (14) defined saturation flow as a function of width of road as in Equation (1).
𝑆 = 180𝑊, PCU/hrg /lane
(1)
Where S, is the saturation flow in PCU/hour/lane and W is the width of road approach in feet.
But the scope of this formula does not cover the full range of road widths, it is valid only above a width of 5.5m. The
same formula has been adopted by the IRC:106-1990, with W in meters and suitable adjustment factors are provided
to account for the effect of left turns and right turns as in Equation (2)
𝑆 = 525𝑊 PCU/hr
(2)
Adjustment factors are applied to account for the effects of roadway, vehicle compositions, turning percentages and
other influencing factors that are ideal to estimate the saturation flow rate. According to TRB, the saturation flow rate
of an approach at a signalized intersection can be calculated using Equation (3).
𝑆 = 𝑆0 𝑥 𝑁 𝑥 𝑓𝑤 𝑥 𝑓ℎ𝑣 𝑥 𝑓𝑔 𝑥 𝑓𝑝𝑝 𝑥 𝑓𝑏𝑝 𝑥 𝑓𝑟𝑡 𝑥 𝑓𝑙𝑡. PCU/hg
(3)
Where,
S = saturation flow rate under prevailing conditions, expressed in vehicle per hour of effective green time in lane
group (veh/h), S0-ideal saturation flow rate which is 1,900 passenger cars per hour of green, N-the number of lanes in
a lane group, fhv-adjustment factor for heavy vehicles in a traffic stream, fw-,adjustment factor for lane width, fg adjustment factor for approach grade, fpp-adjustment factor for the existence of a parking lane and parking activity
adjacent to lane group, fbp- adjustment factor for the blocking effect of local buses stopping within the intersection
area, fa -adjustment factor for area type, frt-,adjustment factor for right-turns in a lane group, flt-,adjustment factor
for left-turns in a lane group.
Calculation of saturation flow rate by Indonesian HCM is given by equation (4)
𝑆 = 𝑆0 𝑥 𝑓𝑐𝑠 𝑥 𝑓𝑠 𝑥 𝑓𝑔 𝑥 𝑓𝑝 𝑥 𝑓𝑟𝑡 𝑥 𝑓𝑙𝑡 PCU/hg
(4)
Where, S-Saturation flow in PCU/hg, S0-base saturation flow in PCU/hg given by 600W,W-Width of carriage way in
m. fcs - adjustment factor city size, fsf - adjustment factor side friction, fg -adjustment factor for approach grade,
fpp-adjustment factor for the existence of a parking lane and parking activity adjacent to lane group, frt-,adjustment
factor for right-turns in a lane group, flt- adjustment factor for left-turns in a lane group.
8
Analysis:
In this study, to calculate the capacity, base saturation flow and adjustment factor for right turning movement and left
turning movement, the following assumptions are made.
1.
2.
3.
4.
5.
6.
8.1
The junctions have an approach width above 5.5 m, as the saturation flow rate formula given by IRC:SP 411994 is valid for width above 5.5 m.
All the right turning vehicles follow a double stream at 3 legged signalised intersections and single stream at
4-legged intersection as per IRC: SP41-1994.
Majority of junctions have no free left turning movements, making it to consider for the saturation flow of
the signalised intersection.
There are no bus stops at or near (within 50 m) the signalised intersections, reducing the saturation flow at
the intersection.
There is no parking provided near the junctions, causing reduction in the saturation flow at intersection.
The PCU values for different categories of vehicles are considered as given by IRC SP: 41-1994.
Field measurement of saturation flow.
The average headway method based on time headway of departing vehicles cannot be used for non-lane based traffic
condition, because in non-lane based traffic flow, headways are difficult to observe, as vehicles do not move in lanes.
Traffic is analysed on the basis of total width of approach and hence, the option of vehicle counting is adopted.
Saturation flow is calculated independently for each observed saturation period and then averaged over observed
cycles. All counted vehicles are added and the sum is divided by saturation period to get saturation flow in vehicles
per hour.
8.2
Methodology
The method used in this study is like the procedure presented in the Chapter 16 of the 2000 Highway Capacity Manual.
The methodology adopted for this research has been shown in Figure (5).
Identification of signalised intersection for study
Data collection
Traffic data by videography
Signal data
Geometric data
Extraction of data and peak
hour identification.
Development of adjustment factors for saturation
flow rate
Saturation flow rate calculation by
1.
2.
IRC SP41-1994 method
New proposed saturation flow model.
Figure 5: Flow chart of methodology adopted for the study
The estimation of adjustment factors for saturation flow rate have been presented in the following sections.
8.3
Effect of lane width on saturation flow rates
Among the geometric conditions, the width of the lane, influences the capacity of the signalized intersections. The
effect of width was studied with respect to the field saturation value and model was proposed. As per the proposed
model the base saturation flow can be found by equation (1) or figure 6.
S0=600*W PCU/hg,
( 1)
where , S0 is the base saturation flow (PCU/hg), and W is the road width (m) .
Base saturation flow ( PCU/hg)
Base Saturation flow (So)
14000
12000
10000
8000
y = 600x
6000
4000
2000
0
0
5
10
15
20
25
Road width (m)
Figure 6: Base saturation flow for road width from 5m-20m
8.4
The effect of right turning movement on saturation flow rates:
Among the traffic conditions, the right turning and left turning movement influences the capacity of the signalized
intersections. The effect of right turning movements was studied with respect to the field saturation value and
adjustment factor have been developed for new proposed model. The right turning adjustment factor(frt) can be
calculated by equation (2) or figure (7)
frt=1+0.17 * Prt
(2)
where, frt is the adjustment factor of right turning vehicles and Prt is the percentage of right turning vehicles.
factor of right turning
vehicles(frt)
Factor of right turn(frt)
1.20
1.15
1.10
y = 0.17x + 1
1.05
1.00
0.0
0.2
0.4
0.6
0.8
% of right turning vehicles
Figure 7: The factor of right turn for different categories
1.0
8.5
The effect of left turning movement on saturation flow rates:
Among the traffic conditions, the right turning and left turning movement influences the capacity of the signalized
intersections. The effect of left turning movement on red (LTOR) was studied with respect to the field saturation
value and proposed adjustment factor for left turning movement (flt) is given by equation (3) or figure(8).
flt=1-0.1*plt
(3)
where, frt is the adjustment factor of right turning vehicles and Prt is the percentage of right turning vehicles.
Factor of left turn (frt)
Factor of left turn
1.00
0.98
y = -0.1x + 1
0.96
0.94
0.92
0.90
0.88
0.0
0.2
0.4
0.6
0.8
1.0
% of left turning vehicle
Figure 8: The factor of left turn
8.6
Saturation rate flow model:
By adopting the above adjustment factors, a new saturation flow model can be developed by using right turn factor,
and left turn factor along with base saturation flow as shown in equation 4,
S= S0 * frt *flt
PCU/hg
(4)
where, S is Saturation flow in PCU/hg, S0 is base saturation flow in PCU/hg ,frt-factor of right turn and flt -factor of
left turn. Comparison of the saturation flow by IRC SP-41 and the proposed model in this study and field value are
shown in figure 9
.
Field value(PCU/hg)
Comparision of saturation flow by IRC :SP41-1994 method & Proposed
saturation model
12000
10000
8000y = 0.9535x + 1000
R² = 0.5436
6000
4000
y = 0.7898x + 1000
R² = 0.5186
2000
0
0
2000
4000
6000
Saturation flow(PCU/hg)
8000
10000
Figure 9: Comparison of saturation flow by IRC: SP41-1994 method & Proposed saturation model
From the above graph, the proposed saturation flow model gives slightly better R2 value. This shows that by
introduction of adjustment factors, a better picture of the saturation flow rate at signalized intersections can be
obtained.
9
Summary and conclusion:
This paper studies the saturation flow rate at signalized intersections and some influence of factors. Referring to the
methodology of Highway Capacity Manual, the saturation flow rate estimate model was developed based on the field
data collected in Bangalore city, India. Due to the complexity of adjustment factors, only parts of them are analyzed
such as lane width, right turning movement and left-turning movement were considered. Based on ideal condition, the
base saturation flow rate and adjustment factors are reached. The study obtained the following conclusions.
a) In this paper ,30 signalized junctions in Bangalore urban area with varying traffic parameters were studied.
b) In new proposed saturation flow model, the base saturation flow(So) were calculated as a function of width
of road, given by S0=600*w PCU/hg, where,S0 is the base saturation flow and W is the width of road in m.
c) The adjustment factors frt is given by frt=1+0.17*prt, where, frt -factor of right turn and prt -percentage of
right turning vehicles
d) The adjustment factors frt is given by frt=1-0.1*plt, where, flt -factor of leftt turn and plt -percentage of left
turning on red(LTOR) vehicles.
e) The proposed model provides the better R2 value compared to the IRC: SP41-1994 method.
f) This shows that by introduction of influencing factors with base saturation flow can give better picture of
field conditions at signalized intersections, especially under heterogeneous traffic conditions of an urban area
in the Indian context.
10
Acknowledgement:
The first author would like to express her sincere thanks to the Police personals in TMC, Bangalore for helping in
obtaining the field data from the proposed/identified junctions.
11
References
1. Highway Capacity Manual. USA : Transport Research Board, 2000.
2. Indo-HCM— India’s Highway Capacity Manual Project. Kayitha Ravinder, Senathipathi Velmurugan , and
Subhamay Gangopadhyay. s.l. : Transport Research Board, 2014, Vol. 295.
3. IRC:106-1990. Guidelines of capacity of urban roads in Plain areas. New Delhi : Indian Road Congress,
1990.
4. SP:41, IRC. Guidelines for the design of at grade intersections for rural and urban areas. New Delhi : Indian
Road Congress, 1994.
5. Highway capacity Manuals for Asian conditions. Bang, Karl-L. Indonesia : Joumal of the Eastenl Asia
Society for Transportation Studies, 1995, Vol. 1.
6. Study on the Saturation Flow Rate and Its Influence Factors at Signalized Intersections in China. Chang-qiao
SHAO, Jian RONG, Xiao-ming LIU. Sweden : Procedia Social and Behavioral Sciences , 2011, Vol. 16.
7. Some factors affecting signalised intersection capacity. Stokes, Robert W. USA : Institute of Transportation
Engineers,, 1989.
8. Self-organizing map of artificial neural network for defining level of service criteria of urban streets. Smruti
Sourava Mohapatra1, Prasanta Kumar Bhuyan. s.l. : International Journal for Traffic and Transport
Engineering, 2012, Vol. 2(3).
9. Effect of heterogeneous traffic on saturation flow. Momin Safabanu Fazalmohammed, H.K.Dave. s.l. :
International Journal of Engineering and Technical Research (IJETR) , , 2014, Vol. 2(3). ISSN: 2321-0869.
10. Development of Saturation Flow Model for Mixed Traffic on Urban Arterial Roads, Intersection . N.G.Raval,
Subhas Kumar C Singh. 2014 | , s.l. : IJSRD - International Journal for Scientific Research & Development,
2014, Vols. 2(3), . ISSN (online): 2321-0613.
11. Estimation of right-turn adjustment factor at signalized intersections for Malaysian traffic conditions:
Considering turning radius and proportion of turning vehicles . W. H. Wan Ibrahim, Q. S. Hossain,L. V.
Leong. Malaysia : Advances in Transportation Studies , 2007.
12. Traffic management centre. Bangalore : s.n.
13. Road Note 34: A Method of Measuring Saturation Flow at Traffic Signals. UK : Road Research
Laboratory.
14. Traffic Signals. Cobbe, Webster F.V. and B.M. UK : HMSO, 1966.