DELAYS AND sr?pLUTI°NS T° MIN™IZE 
DELAYS AND SAFETY RISK AT DEHIWALA
flyover
Buddhika Yahampath 
Department of CM Engine^
(email: buddhika.bcrox@gmail.com)
Saman Bandara
Department of Civil Engineering, University of M 
(email: bandaras@uom.lk)
Loshaka Perera 
Department of Civil Engineering, University of Moratuwa 
(email: perera.loshaka@gmail.com)
oratuwa
:
Abstract; Traffic congestion is one of the most imperative problems in almost all countries 
Flyovers are usually used all over the world to address the traffic congestion, especially at 
intersections. Advantages of grade separation are; less interruption to traffic flow, higher speed and 
reduces the scope for accidents. In Sri Lanka also flyovers are used to address the delavs and the 
safety risk at highly congested intersections. Dehiwala intersection was also replaced by a flyover in 
order to eliminate the traffic congestion. However, congestion and accident trends have not been 
reduced yet. There are many factors that contribute to increased congestion levels especially during 
peak periods and the issues behind the increase in accidents at Dehiwala junction. Engineering 
solutions based on traffic and highway engineering principles can effectively be used to improve the 
traffic flow and reduce traffic accidents at this location. Accident data analysis, speed and travel time 
surveys were used to compare the conditions at present and before construction of the flyover. 
Moreover vehicle movement pattern, road geometry and adjacent land use activities can be used to 
evaluate the impact to the Dehiwala flyover. Geometry of the road need to be improved as it will not 
generate significant speed difference between two adjacent sections as well as not generate bottlenecks 
at the entrances of the flyover. Stopping of buses within 75m from the junction and vehicle parking 
within 50m range from the intersection need to be discouraged by using regulatory methods.
\
;
Flyovers, Engineering solutions, minimize delays, safety risks, accidents.Keywords:
Dehiwala flyover is the second such flyover in 
Colombo. Prior to this solution Dehiwala 
junction has undergone a number of traffic 
management options varying from a 
roundabout to signalized roundabout and then 
a signalized intersection and finally a flyover. 
However, the congestion issues have not been 
properly solved.
Objectives of this study are to identify the 
factors contributing to increased congestion 
levels especially during peak periods and the 
factors contributing to increase in accidents at 
Dehiwala junction and propose engineering 
solutions based on traffic and highway 
engineering principles which can be used to 
improve the traffic flow and reduce traffic 
accidents. Conditions at present and before the 
introduction of the flyover can be compared by 
using accident data analysis, speed and travel
1. Introduction
Traffic congestion is one of the most vigorous 
topics in traffic engineering, irrespective of 
whether in a developing or developed country. 
In all situations where capacity cannot be 
provided for the peak demands, waiting and 
delay are inevitable. Avoiding traffic 
congestion is an important aspect in traffic 
engineering. Economy of a country depends 
on the quality of the road networks.
Different traffic management approaches have 
been used for a long time, based on the 
appropriateness to reduce the issue. Grade 
separation is one final solution that allocates 
for each road at various heightsvertical space 
in order to avoid disruption to the traffic flow
roads areat the junctions where two or more 
crossing. In Sri Lanka, flyovers 
introduced to reduce traffic congestion 
accident tendency at road intersections.
recently 
and
are
97
evaluation of the functions at the intersection 
transition, terminal, and the roadtime surveys. Figure 1 shows the plan view of 
the junction. All dimensions in meters. such asgeometry particularly the gradient and width 
critically before decision making on any
flyover project.
and Hassan (2005) conductedMisaghi
experiments on two-lane rural highway to 
research the relationship of geometric features 
the operating speed consistency,
r
3
£I *
t anddiscovering that driver's perception of the 
road rather than the designer's was the main 
factor on speed decision. Effect of visual range 
on driving speed on low-grade highway 
studied by Wang, Fang and Wang (2013). It is 
normally believed that the better the visual 
the safer the driving environment 
researchers
-iBus StopF
27 ::
Hill Stra«tStation Rood
==3===-
1 &yA_-8.3-1 was
I
range, 
becomes.
mentioned that this is not always the case. If 
the visual range is too large, it easily leads to a 
big speed difference between the adjacent 
parts. So the length of visual range should 
correspond with the design standard of the 
whole road. Moreover they mentioned that the 
significant speed difference is definitely 
unfavorable to road safety. Therefore, they 
have recommended limiting visual range of 
80m to prevent speeding. Visual ranges of 
road sections which are adjacent to the flyover 
need to be compatible, as it helps to reduce the 
speed difference between flyover section and 
adjacent sections.
73
haveHowever
t Bus Stop
2
Figure 3: Plan view of the junction
Literature review2.
Peiris & Fernando (2011) have studied the 
appropriateness of a flyover as a traffic 
management measure. They found that 67% of 
accidents were occurring on the flyover. Buses 
and heavy vehicles contribute to 74% of 
accidents at the ends of the flyover. They have 
identified that the monthly average of 
recorded accidents in 2007 were 10 and 
average increased up to 36 by 2010. They have 
mentioned that the disadvantages of the 
flyover are comparatively higher than its 
expected benefits. Less co-ordination among 
relevant administrative bodies and the shared 
responsibility between road development 
agencies, management bodies as well as the 
local authority in pre and post assessment of 
the performance of flyover at the location was 
not done properly were some of the other 
observations by them. Peiris & Fernando 
(2011) have mentioned about a sudden speed 
drop at the flyover. They have identified that 
the obstructive design of the flyover at the 
approach level blocks the visual path of the 
and the lack of overtaking opportunities 
exist under heavy traffic condition. Also the 
functional efficiency and the land use activity 
patterns were not considered at all in these 
mega projects as well as feasibility studies 
were not done. Eventually they have 
recommended that the land use compatibility 
evaluated with the transport system before 
m roduemg costly traffic management tools 
such as flyovers. They have also suggested
Meadows(2010) has cited that the inadequate 
width of both sides of the road and trading 
activities on the sidewalks are reasons for the 
issues. ITe has mentioned that the "Marine 
Drive" project from Moratuwa to Dehiwala 
would be a good solution for the existing 
issue. Zilioniene and Vorobjovas (2011) 
investigated the horizontal alignment of 30 
regional roads, finding that sight distance, is 
one of the safety assessment indices, directly 
affected the acceleration and deceleration 
performance of vehicles on long tangent 
section.
Surrogate safety measure for evaluating rear- 
end collision risk related to kinematic waves 
near freeway recurrent bottlenecks 
studied by the Li, ct ah, (2013). They 
mentioned that the likelihood of rear-end
users
was
collisions is highest when traffic approaching 
from upstream is at capacity state while traffic 
down-stream is highly congested. Abdel and 
Pandc (2005) mentioned that the speed 
difference between upstream and downstream 
locations and the wereaverage occupancy;
98
identified as contributing factors for collision
risk.
Shankar, Mannering, & Barfield (1995) 
mentioned that the section with steeper 
maximum upgrade between any two sections 
will experience a greater number of rear-end 
and other same-direction accidents. In 
addition, rear-end accidents will increase 
substantially if the maximum grade exceeds 
2% in that section. The impact of grades is 
reversed in the presence of downgrades. Much 
of the effect of the higher braking distance on 
downgrades appears to be offset by the visual 
impact of brake lights warning drivers of the 
potential slowing of vehicles ahead. In 
contrast, drivers are unlikely to use brakes on 
an upgrade which eliminates a critical warning 
sign of speed reductions. And also they have 
mentioned that the number of rainy days in 
the month decrease sideswipe and rear-end 
collisions and increased fixed object collisions
3.2. Objectives of introduction of flyover
The purpose of construction of this fly-over is 
to eliminate the traffic delay in Galle Road
(AA002) at the Dehiwala Junction. (Dehiwala 
flyover, 2011)
3.3. Impacts of the flyover
Accident tendency and congestion level has 
increased at the intersection. This was 
confirmed by the Dehiwala Traffic Police 
Division and they had to allocate more officers 
the flyover. Trading activities 
also have influenced and previous researches 
(Peiris & Fernando, 2011) have mentioned 
about it.
on duty near
3.4. Data collection
The location was observed for a few hours 
during the peak periods of the week days and 
special features and issues of the flyover were 
identified. Intersection and entrance to the3. Methodology
flyover from both ends were observed. About 
lkm distance along the A002 road was 
observed and vehicle and
3.1. Methodology of study
passenger
movement patterns were recorded for 
supplementary studies. Dehiwala police 
station was visited and accidents records were
Identify the objectives of the introduction 
of flvover
referred. Police officers, road users, 
pedestrians, sellers and shop owners were 
interviewed. Accident data was obtained from 
the traffic police headquarters and Dehiwala 
police station Newspaper articles, videos 
relevant to the Dehiwala flyover were also 
collected. Breaking of rules and mistakes 
identified by examining recorded videos.
Study the impacts of the flyover in 
various aspects
*
Literature survev were
Data Collection Observation and analysis4.
Observation
Gathered data were analyzed to identify the 
variation of the accident patterns, driver 
behavior, and pedestrian movement patterns 
decide the justification of
DInterviews
Accident data/notice reports in order to 
observations.
Survevs
4.1. Driver's behavior patterns
most sections of
Other sources
£ Drivers keep a low speed on Galle Road except for those sections with good visual range where it is easy to lead drivers to 
raise speed to a high value. The situation 
becomes much worse when a bad vision 
alignment follows the good vision section^ 
This situation occurs near the Dehiwala \ 
flyover. Vehicles travelling on Galle Road, A 
especially during night time, travel at higher
Analvsis the condition of flvover
t
Comparison of before and after
i
Conclusion and recommendations
Figure 2: Methodology of study
99 \
■ C-.
selected to study the accident patterns. 
Following parameters were considered for the 
alysis. Severity, accident frequency vs. time, 
lighting condition and location 
considered. Flyover was opened for public on 
21st of October 2009. Before the construction of 
flyover and after the construction of flyover 
were considered to identify the safety situation 
in the area.
4.3.1. Accident frequency in each year
wasspeeds. But due to the visual obstruction at the 
junction they are vulnerable to accidents. 
Especially travelers, who are not the daily 
commuters, can be victims of accidents. Most 
of the time they collide with other fixed 
objects.
During the night times, drivers follow the 
white line which separates the two lanes on 
left side of the road and without notice center 
median appears in front of them in such a way 
that there is hardly any time to avoid the 
collision. Most of the times three wheelers and 
motor cycles use sidewalk for travel. It is also 
observed that vehicles stop on the pedestrian 
crossing during red light. Pedestrians are 
compelled to deviate their path causing 
accidents. During the peak hours vehicles have 
to follow a stop and go type pattern. If they do 
not maintain adequate distance between two 
vehicles it collides with the rear end of another 
vehicle which is travelling ahead of it. When 
drivers need to go to shops, most of the times 
they tend to stop their vehicles on both sides of 
the road, underneath the flyover. It causes 
reduction of limited available width of the 
road. Due to that reason, a two lane road 
becomes a single lane road causing bottle 
necks.
an
were
Accident tendency has gradually increased 
after the introduction of the flyover according 
to the figure 3. Difference between accident 
frequencies of the year 2009 and the year 2010 
is substantial. Before construction of the
flyover accident frequency per year was about 
125. It has increased to 184 in the year 2010. 
Number of accidents that occurred during the 
2012 are almost double the number ofyear
accidents that occurred during 2008.
300 258
250 1
194184200
1150 134
'£ 100
110m •!m- I
4.2. Geometry and other features 34
■-.J.0Dehiwala junction is a heavily populated 
junction and which involves so many trading 
activities such as large shops, small hotels and 
traders on side walkways. There are many 
byroads directly connected to the A002 road. Figure 3: Total accident frequencies in each 
year
4.3.2. Accident frequencies in each hour of 
a day
Time of accidents and frequencies were plotted 
considering data before and after construction 
of the flyover. Same pattern of accident 
distributions were indicated by both graphs. 
Majority of accidents took place during peak 
hours. Normally 10 am to 2pm and 4 pm to 
9pm are high accident frequency hours. A 
drop of accident frequencies 
during 3pm to 4pm (Figure 4). During that 
time period, number of road users is 
significantly less than the peak hours. It cause 
reduction of accidents. But most interesting 
fact is 79% of fatal accidents have occurred 
during night time after construction of the 
flyover. Before 
67%.
Arrangement of the bus stop is another reason 
for increased congestion. Buses are stopping 
closer to the junction to get passengers. 
Passengers also stop at the intersection to get 
in to the buses instead of going to designated 
bus stop which is positioned about 40m away 
from the intersection. During the peak hours it 
generates long queues. Gradient of the flyover 
also cause to reduce the speed of the vehicles. 
Lack of overtaking opportunities and 
inadequate width of the road also induce 
traffic delays. be identifiedcan
n 4.3. Accident Data analysis
Suitable area and the road section for analysis 
was selected such a way that it includes the 
Dehiwala flyover. According to the police map 
Node no 662802 contain the Dehiwala junction. 
Range of 3 km distance along the A002 road
■:!
1
construction of flyover it was
::1
100
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2019 
I 18 13 1318£
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9 15
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15 1514Z
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6 660
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TIME
Figure 6: Severity vs. accident percentage after 
construction of flyover
4.3.4. Accident frequency along the road
4: Accident frequency vs. time before 
struction of flyover
Figure
com
Percentage severity in each accident4.3.3. 
category
Most of the accidents have concentrated to the
vicinity of the flyover as shown in Figure 7. 
Range within the 500m either sides of the 
junction was the most crucial sections and 
large number of accidents has happened 
within this section. Same type of accident 
distribution can be seen before and after
□ Grievous□ Fatal
□ Non-grievous □ Damage only
V construction of the flyover. Most prominently, 
accident frequencies at that region have 
increased significantly after construction of 
flyover. Accident trend of other locations (at 
least 500m away from the junction) along the 
Galle road have not changed significantly after 
introduction of the 
frequencies within the flyo 
while accident frequencies of other locations 
from the junction have remained almost 
as before. The most critical location is the 
In addition the location
Accidentflvover.
have increasedver
away
same
Dehiwala junction, 
about 500m away from the junction towards 
south (Mount Lavinia) represent another
Figure 5: Severity vs. accident percentage 
before construction of flyover
Percentages of fatal, grievous, non-grievous 
and damage only accident percentages for 
period before 2010 are shown in Figure 5. 
Above statistics have been changed after 
introduction of the overpass to the junction. As 
shown in Figure 6 it can be seen that fatal 
accident percentage has increased. While 
grievous accident percentage has reduced after 
the introduction of the flyover at Dehiwala 
junction. Non grievous accident percentage 
also has reduced during latest years. But 
damage only accidents 
significantly.
critical place.
Accident type vs. frequency4.3.5.
vehicle in position"Rear-end crash hitting a 
for going straight ahead" is the most common 
type of accidents which have occurred at this 
area in every year irrespective of the 
truction of the flyover. "Overtaking to the 
right", "Over taking to the left", "Shifting lane 
or merging to the right", "Shifting lane or 
rging to the left", "With pedestrian entering 
the road section from the left sidewalk, 
shoulder etc.", types "With pedestrian 
entering the road from the left prior to 
intersection", "With pedestrian entering the
cons
m
increasedhave
! 101
a Fata!■ Grievous Location of the 
flyover
■ Damage Only ONon-Grievous
45
40
35
30
Bz 25
U-J
a 20
UJ
ijjjl j Jj
oooooooo o°Soo°°°§^2?20oq
li||i||||l8gsi§|§§§&ss2sasSs
15
10
5 8 v . P, fl.. J 8 i Pill, ml PIP0 * *
LOCATION
Figure 5: Severity vs. location after construction of flyover
section from the left sidewalk, shoulder etc.", 
"With pedestrian entering the road from the 
left prior to intersection" and "With passenger 
being hit when hanging outside vehicle" 
because pedestrians are more likely to be 
injured during an accident. Damage only 
accidents are generated by the other accidents 
types such as "Other head on crash", "Rear- 
end crash hitting a vehicle in position for 
going straight ahead"
road from the left after an intersection", "In 
conjunction with overtaking and "Other 
crashes with fixed object" 
accident types.
are common
4.3.6. Location vs. severity
183500, 183550, 183100, 183000, 183400 and 
183700 are the northing coordinates of the 
identified dangerous locations (Figure 7). Fatal 
accident frequencies have increased after the 
introduction of the flyover as a traffic 
management solution. Grievous accidents are 
also more likely to happen closer to the 
junction after construction of the flyover. 
Above described pattern can be identified in 
2008 and 2009. But after the introduction of 
flyover number of accidents has increased. 
Most importantly damage only accident 
frequency has increased significantly within 
the region of 500m from the junction to the 
both directions. In 2008 and 2009 most number 
of accidents has happened between northing 
183000 and 183550. In 2010 and 2011 similar 
distribution can be seen while frequency has 
significantly increased. But in the year 2012 
majority of damage only accidents have 
happened between the northing 183200 and 
183900. That means accident distribution has 
spread either side of the flyover.
4.3.8. Street lighting condition
Table 2: Severity and existed lighting 
condition
lighting condition
nigh limp 
roper 
sheet 
lighting
night,goo 
d street 
lighting
night,no
street
lightingseverity daylight
fatal 33 00
grievous 010 02
0non grievous 
damage only
32 08
0146 143
In 2012, accident severity and the existed 
lighting conditions 
According to Table 1 considerable number of 
accidents also occurred under
shown in Table 1-are
lighting
condition. However, fatal accidents have 
occurred under good street lighting conditions 
at that night. Number of fatal accidents that 
uired during night and day time is similar. 
Generally the number of vehicles travelling at 
night is less than the number of vehicles 
travelling at the day time. Hence accidents
no
4.3.7. Accident type vs. severity
Most of the fatal accidents happened due to 
accident types which involves pedestrians 
such as "With pedestrian entering the road
occ
1
__102d
ill
possibility is higher during night relative to 
day time. vehicles to reduce while driving upward dn-Khon. are unlikely to u*hJkes on
pgrade, which cause to increase the risk of 
meeting with a rear end collision.
Drivers tend to 
visual
4 3.9. Days of week
speed up their vehicle when 
range is large. But situation 
worse if bad vision section follows 
veison section. Road
Figure 7 illustrates the accident occurrences in 
each day of the week. Lesser number of 
accidents has occurred on Saturday and 
Sunday where traffic flow is relatively low. 
accidents have happened on the
can get 
a good 
sections which are 
adjacent to the Dehiwala flyover contain good 
vision sections specially during off peak times. 
Because of that, drivers increase their speed on 
that sections. But if they not reduce the speed 
significantly closer to the flyover, it can cause 
accidents. Because visual range of the flyover 
section is lower than the adjacent sections. 
Wang, Fang, & Wang in 2013 recommended to 
maintain speed diffrences within some range 
to avoid safety issues. Especially during the 
night times, drivers follow the white line 
which separates the two lanes on left side of 
the road. But due to the high speed of the 
vehicle and visual obstructions at the location, 
drivers can't identity the fixed objects such as 
center median, sign boards or concrete guard 
rail. Specifically non-daily commuters are 
facing huge problems due to lack of 
knowledge about the location and most of the 
time they become the victims.
More
Thursdays and Fridays as compared to other
weekdays.
0.8
0.6
0 0 0 0 0 0
£ 0
u- Sun Mon Tues Wed Thrs Fri Sat 
day day day day day day day
Day
Figure 8: Day of the week vs. accident 
frequency People tend to stop their vehicles at both sides 
of the road, underneath the flyover, causing 
reduction of limited available width of the 
road. Due to that two-lane road becomes a
Conclusion5.
single lane road causing bottle-necks. Further 
it reduces the visual range of the drivers and 
creates the uncertainty in their mind as side 
walkways are congested during the peak 
hours. Random crossing of pedestrians 
also cause accidents. Some sign boards 
missing at the junction. Because of that road 
unable to identify important features 
of the road. Further, bad lighting conditions at
and visual
Safety of the road users has been significantly 
influenced by the flyover. Delays have been 
induced due to the high congestion especially 
during peak hours. Expected benefits have not 
been achieved yet from the flyover project.
Reasons for accidents and delays
Geometry of the road has induced significant 
impact to the traffic flow especially during 
peak hours. Most importantly bottle necks 
created at the entrance of the flyover due to the 
existing shape of the road. Inadequate width 
of the road involves increasing the traffic 
congestion and the accident frequency. Lack of 
overtaking opportunities is another reason. 
Especially during congestion period smaller 
vehicles try to reduce travel time by 
using sidewalks.
According to the Shankar, Mannering and 
Barfield (1995), rear-end accidents will increase 
substantially if the maximum grade exceeds 
2% in a section. Gradient of Dehiwala flyover 
is also at a higher range. It causes the speed of
can
are
5.1.
users are
the entrances to flyover 
interferences magnify the above issues.are
^siXT^rB Krsrsff-
would have reduced to certain extent if busses 
are stopped at the established bus stop.
Recommendations
even
6.
Of the road has to be changed in 
that it reduces the occurrence of 
of the tlyover. 
of the road
Geometry
bottlenecks at the entrances bottlencc thfi yisual ranges
Alignment an 103
http://print2.daiIymirror.lk/features/featm-eg
/4090.html
Misaghi, P., & Hassan, Y. (2005). Modeling 
Operating Speed and Speed Differential 0n
Two-Lane Rural Roads. Journal nfTransportation, 232(6), 408-417. *
Peiris, O. V., & Fernando, K. D. (2011). An 
examination of the appropriatness of flyover as a 
traffic management measure to the road
intersection of Sri Lanka. Town and Country planning, University of Moratuwa. y
Shankar, V., Mannering, F., & Barfield, W 
(1995). Effects of roadway geomatries and 
environmantal factors on rural freewav 
accident frequencies. 371-389.
Wang, P., Fang, S., & Wang, J. (2013). Effect of 
Visual Range on Driving Speed on Low-grade 
Highway. 13th COTA International Conference of 
Transportation Professionals (C1CTP 2013) (pp 
1175 - 1184). Shanghai, 201804, China: 
Procedia - Social and Behavioral Sciences.
Zilioniene, D., & Vorobjovas, V. (2011). 
Analysis of Driving Conditions 
volume Roads in Lithuania Assessing 
Geometry Parameters of. 8th International 
Conference Environmental Engineering, (pp 
1254-1260). Vilnius, Lithuania.
sections need to be improved as those do not 
generate significant speed differences between 
adjacent road sections especially at the 
approaches to the flyover. Vehicle parking 
within 50m range from the intersection needs 
to be discouraged otherwise it causes to induce
drivers mind andthe uncertainty in the 
increase the visual obstructions to the drivers.
the bottle-necks at the 
the bus traffic near
And also it creates 
Sincejunction.
intersections impede with the traffic flow, it is
should bedesirable that the bus stop 
positioned at least 75m away from the junction 
for passengers to disembark at safe places such 
as curbs or islands. (Kadiyali, 2007). And also 
rules and regulation need to be prepared for 
enforce the road users to follow that rules. Sign 
boards need to be provided at least from 500ni 
y from the flyover indicating the existence 
of flyover, road crossings, and fixed objects. 
Further, adequate lighting needs to be 
provided at the approaches to the flyover. 
Curbs and required safety precautions have to 
be taken to increase the safety of the 
pedestrians. Appropriateness of the timing of 
the traffic signals should be checked. If timing 
of the signals are not appropriate it generates 
delays, significant increase of accident
end collision),
awa
on Low-
more
frequencies (especially 
increase the use of less adequate routes as road 
attempt to avoid the traffic control 
tend to neglect the
rear
users
signals and drivers can 
important traffic signals.
References
Abdel, A., & Pande, M. (2005). Identifying 
crash propensity using specific traffiespeed 
conditions. Journal of Safety Research, 97-108.
Dehiwala flyover. (2011, 07 27). Retrieved from 
Road development Authority 
http://www.rda.gov.lk/supported/flyovcrs/ 
dehiwala/dehiwala.html
Kadiyali, L. R. (2007). Traffic engineering and 
transport planing. Delhi: Kanna publishers.
Li, Z., Ahn, S., Ragland, D. R., Chung, K., 
Wang, W., & Yu, J. W. (2013). Surrogate safety 
measure for evaluating rear-end collision risk 
relatedto kinematic waves near freeway 
recurrent bottlenecks. Accident Analysis and 
Prevention, 52- 61.
Meadows, W. (2010, 02 19). The awesome 
Dehiwala flyover. Retrieved from Dailymirror:i
. >.1
104i1 •