Improving performance of roundabout [621538]

Improving performance of roundabout
intersections by increasing traffic speed
Marinela Ință1*,
1Lucian Blaga University of Sibiu, Industrial Engineering and Management Department, 550025 ,
Victoriei, no.10, Sibiu , Romania
Abstract: Streamlining of urban traf fic influence nowad ays almost
everything about our modern life. The main effect, which we all feel as the
daily traffic participants, either as pedestrians or vehicles drivrers, is travel
time to work and back. Roundabouts intersections have a number of
advantages over intersections with traffic lights, they reduce the severity of
accidents, traffic speeds to provide an "calmer" road environment. So,
operating performance of roundabouts are complicated, influenced by the
confluence operations. The paper aim s at analyzing the current situation of
traffic in two crowded roundabouts in Sibiu, comparing the results with the
original data and proposing solutions to improve traffic conditions and
especia lly traffic speed. The obtained results will be the basis for
improving the overall capacity of roundabouts.
1 Introduc tion
1.1 Backgrounds
Transportation is an important factor in the development of society. Besides transport have
a positive impact on economic development, they attract all the negative effects such as
pollution of the environment, traffic congestion, accidents, etc.
Sibiu is one of the most prosperous cities in Romania and receives one of the highest
rates of foreign investment. Thus, besides the traditional industrial city (engineering
industry – Bilstein Compa, clothing industry – Mondostar, food industry – Scandia, after
1995 were created new companies in the field of automotive (Continental, Kuhnke relays,
SNR bearings, Takata, Siemens).
With the development of industry, also population grew in S ibiu. As seen in the chart
below, Sibiu's population has increased more than 11 times in a period of 163 years since
they achieved first statistical data collected in censuses.
With this population growth, increased automatically and their need to travel. So, the
number of registered vehicles has increased considerably in Sibiu, plus the ones in transit .

* Corresponding author: [anonimizat]

Fig. 1. Evolution of the population of Sibiu
If in 1985 the daily average annual vehicles circulating in Sibiu was 846, in 2010 their
number doubled to 1752, and forecasts show that by 2025 the number of vehicles that will
move could increase up to 3254, according to data provided by the National company of
Motorways and National Roads in Romania [1].
1.2 Historic
The spread of roundabouts began when British engineers have redesigned circular
intersection in mid 1960 and Frank Blackmore invented the mini roundabout to overcome
capacity limitations of movement and protection measures [2].
Unlike circular intersections, roundabouts operate with rules to give prior ity to traffic
already in the intersection and the traffic that leaves the intersection and the driver removes
much of the confusion associated with circular intersections and waiting time for the drivers
to intersections with traffic lights. Having roughl y the same size as the intersections marked
with the same capacity, roundabouts are significantly smaller in diameter than most circular
intersections, separates entering and exiting traffic of the intersection with sites developed
for pedestrians to encou rage driving at lower speeds .
Sibiu is in full economic development, leading to the development of transport and
automaticaly to the emergence of traffic congestion. This can be seen in the peak hours in
the central areas and in areas of entry / exit in the city.
One of the main arteries of the city is the road of Alba Iulia that connects the central and
western industrial area (exit to Sebes – Alba). This artery is made up of several types of
intersections including roundabouts and large flow of vehicles c reates problems in terms of
fluency, resulting in increased congestion level.
2 Data collection
The biggest traffic congestion occurs on the road in the area formed by two roundabouts
(Alba Iulia – Autogarii and respectively – Alba Iulia – Maramureșului – Malului). This is
due to the following reasons:
– A zone of economic interest (linking with the Western industrial area, along artery
are points of interest such as Banca Carpatica, store Lidl, DM, Kaufland, Peny
market, gas sta tions, hotels, restaurants, et c;

– Main access route to Sibiu Inter national Airport;
– Positioned on both sides of the river Cibin.

Fig. 2. Alba Iulia – Autogării Fig. 3. Alba Iulia – Maramureșului – Malului
Table 1 . Geometric elements of intersections
Roundabout Alba Iulia –
Autogării Roundabout Alba Iulia –
Maramureșului – Malului
Inner radius 9,5 m 8 m
Outer radiu s 14,5 m 16 m
Number of annular strips 2 2
Width of the annular band 4-5 m 4 m
Entry lanes 2 lanes on each arm 2 lane on each arm
Entry lanes width 3,5-4 m 3-4 m

Although over time they have made improvements in terms of structure and type of
interse ction, and today can be seen queues and delays. Queues formed both morning and
afternoon when most city residents are going or returning from work or school units, as
well as on weekends when they go to supermarkets.
Traffic data were collected using tradi tional through manual spot counting of vehicles,
then counting their record sheets per the category they belong.
These were collected through an observer team during a week to cover variations in
traffic. The length of data collection was 15 minutes, three times per day for each junction.
Under the traffic data, collection was followed:
– registration number of cars on each band ;
– pedestrian traffic record ;
– the length of the queue ;
– the number of cyclists ;
– frequency of the means of public transport .
It was veri fied that during data collection not appear major errors which , then to affect
results because errors can lead to misinterpretations of traffic conditions. Before data
collection there were studied the twoo roundabouts to determine the points of maximum
visibility to allow observation of all intersection arteries.
Because traffic flows have a variable component, indentificating the vehicles in different
categories was used for mixing standard vehicles through equalization coefficients. These
coefficients represent the ratio of occupying road space by dynamic physical vehicle and
vehicle occupancy standard – equal to unity – that is the car.

Thus, Figures 4 show the average standard vehicles volumule in the twoo roundabouts
(Alba -Iulia- Autogării noted wih R1, Alba Iulia – Maramureșului – Malului noted wih R2)
collected during one week.

Fig. 4 . Hourly and daily distribution of traffic volume data collected for two roundabouts
3. Traffic Data Analysis and levels of service (LOS)
After data collection , first were analyzed conflict points for the twoo intersection. Total
conflict points of intersection being 33 and 35 respectively, divided into:
– divergence points – 9;
– convergence points – 7;
– crossing points – 17 and 19 respectively (7 points known crossing p oints vehicle –
vehicle, 10 crossing points vehicles – pedestrian).
These data were then compiled Microsoft Excel data sheets calculation along with each
roundabout, scheme, based on literature formulas w ere calculated and analyzed average
speed on each arm, peak h our factor of each intersection .
3.1 Road traffic speed
Velocity is the main parameter of the road, because it characterizes purpose of travel and
depends on the technical characteristics of vehicles, road network and capacity of vehicle
drivers.
tlV
(km/h) (1)
where :
l- length of road sector examined ;
t – time in which the road was tranzited.
Average travel speed for every roundabout in each direction is shown in the figure below.
The difference between the speed between the three arms is caused by different diameters
of the bands and confluence actions [3].

Fig. 5 . Average velocity of vehicles (km/h)
These values indicate that vehicles confluence influences vehicle speed on band 1.
Confluence vehicles have lower speeds beca use they usually decelerate before entering the
roundabout.
3.2 Peak hour factor (PHF)
Peak hour factor in urban areas generall y range between 0.80 and 0.98. Values lower than
0.80 means greater variability traffic flow, and values close to 0.98 represents small
variations in traffic flows. Peak hour factor over 0.98 indicates a high volume of traffic.
Following application of the formula r esults for the first roundabout a peak hour factor
of 1.23 and respectively 1.07 for the second sense.
To validate values obtained it were modeled and simulated with software Synchro
Studio the twoo roundabouts junctions [4]. The sofware displays related repor ts of:
intersection capaci ty utilization (ICU) , level of service (LOS) , delays of vehicles, total
number of stops, travel time, HC, CO and NOx emissions and fuel consumption (Table 2
and Table 3 ).
Table 2 . Str. Alba -Iulia & Str. Raului Perfor mance by movement
Movement EBL EBT EBR WBL WBT WBR NBL NBT NBR
Delay / Veh (s) 13.3 14.4 15 343.7 288.4 208.7 578.3 702 474.3
Stop/Veh 0.83 0.78 0.52 1.5 1.25 2.07 0.3 0.26 0
Avg Speed (kph) 22 21 21 5 4 14 2 2 2
Fuel Used (l) 0.3 5.1 1 2.1 7.9 2.2 2.9 4.2 0.7
HC Emissions (g) 0 15 2 0 22 1 5 0 0
CO Emissions (g) 11 356 57 43 435 67 103 67 11
NOx Emissions (g) 2 50 7 3 38 7 5 3 0

Following the completion of simulations of roundabouts usi ng the program Synchro
Studio, you may notice an ability to use the intersections of over 100% and a level of
service H, therefore helps to create tails (congestion), low level of comfort and increased
risk of accidents.

Table 3 . Alba -Iulia & Str. Autogari i Performance by movement
Movement EB WB NB SB
Delay / Veh (s) 3.7 6.1 11.6 21.4
Stop/Veh 0.27 0.29 0.67 0.65
Avg Speed (kph) 22 27 8 14
Fuel Used (l) 3.2 2.8 0 3.7
HC Emissions (g) 9 12 0 15
CO Emissions (g) 198 279 1 315
NOx Emissions (g) 29 38 0 45

H level of service (LOS H) describes operations with control delays excessive, over 80
seconds to a vehicle. This level is considered unacceptable by most of the drivers, often due
to over saturation of traffic that occurs due to excedes traffic capacity of intersections.
These delays occur with a ratio V /Q great.
In other words, utilization capacity exceeded intersections and the traffic is not
conducted in the normal range. Because the infrastructure does not allow major changes,
only improvements that can be made are in the geometry of the intersection, which can
have a positive influence on the traffic flow of the twoo roundabouts.
4 Methods of traffic decongestion
4. 1 Changes t o the geometry of intersections
4.1.1 Roundabout Alba Iulia – Autogării
As a solution to increase traffic speed in the roundabout, they have made improvements to
the intersection geometry, namely:
– arm Alba Iulia (the center) were added two lanes, one of which is exclusive left
turn, and the other for, right turn hav ing a length of 50 meters;
– arm Autogarii added an exclusive right turn band ;
– arm Alba Iulia (to Turnisor) was added an exclusive lane for left turn with 20
meter.

Fig. 6. Roundabout Alba Iulia – Autogării proposed solution

As seen intersection capacit y utilization rate was reduced to 90% which means that
traffic flows in normal parameters, and total delay time at the intersection was reduc ed by
cca 40% reaching from 275. 6 to 164.1 seconds.
4.1.2 Roundabout Alba Iulia – Maramureșului – Malului
And our second roundabout noted that capacity utilization intersection exceeds 100%,
which means that the movement is not conducted in the normal range.

Fig. 7. Roundabout Alba Iulia – Maramureșului – Malului initial solution
And if this intersection were pe rformed only changes the geometry and observed decrease
in ICU par ameter of the intersection to 94.6 %.
4.2 Results
Following the implementation of the solutions presented in section 4 were obtained results.
 Roundabout Alba Iulia – Autogării
After implement ing optimization, solution is seen that the average speed of traffic on the
main roads between intervals are 20 to 30 km / h and 30 -45 km / h.

Fig. 8. Roundabout Alba Iulia – Autogării proposed solution
Speed being the smallest on the lanes from the Lidl store exit and the Autogarii is minor
artery arm .
 Roundabout Alba Iulia – Maramureșului – Malului

If the initial situation average speed of traffic on each lane was low, especially on the arm
of Maramures, after implementing the solution is observed that increased velocity being
between 20 to 3 0 km / h.

Fig. 9. Roundabout Alba Iulia – Maramureșului – Malului – the proposed situation
After geometric solutions implementations in the two intersections have reduced the
number of stops and time of crossing. A s disadvantage by implementing optimization,
solution can not say that vehicle emissions have declined, which is higher due to increased
traffic speed. hydrocarbons (HC) reached 18 g, the carbon monoxide (CO) being 1259 g
and the oxide of nitrogen (NOx) is 88 g due to the continuous flow of traffic.
5 Conclusion
After analyzing the data collected from the two roundabouts intersections: Alba Iulia –
Autogarii Road, Alba Iulia -Maramures ului – Raului road, resulted in large volumes of
traffic moving toward th e city center and industrial area with a low -speed, vehicles having
frequent stops and starts, forming queues along the section.
It was concluded from the analysis and simulations performed Synchro that and
pedestrian traffic around intersections influence in a considerable way, because in the area
shopping centers are located.
Simtraffic software confirmed the results of calculations performed in the two junctions,
which resulted in implementing simulation geometric changes. Following the simulation its
implementation, improvement was observed in the range of peak hours’ traffic, reducing
queues and delays the vehicle stops. What has not been modified fuel efficiency, which has
kept the value in both cases.
The results demonstrated that the proposed solutio n increases the average speed of
traffic , reduce queues descongest ing traffic, leading to a better development of it, but they
could see a downside such as emissions made by vehicles, which increased.
References
1. www.recensamantromania.ro/wp -content/uploads/2013/07/sR_Tab_8.xls
2. *** Federal Highway Administration, Manual of Uniform Traffic Control Devices for Street and
Highways , Federal Highway Administration (2009)
3. R. Dowling, A. Kittelson. Active Traffic Management for Arterials NCHRP Synthesis ,
Transportation R esearch Board 447, Washington D.C. (2013)
4. http://trafficware.infopop.cc/eve/forums/a/frm/f/9767036316