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Hrvoje Pilko, Nikola Božić, Nikola Šubić

ANALYSIS OF ROUNDABOUT CAPACITIES IN THE CITY OF ZAGREB USING SWISS METHOD SN 640 024

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ANALYSIS OF ROUNDABOUT CAPACITIES IN THE CITY

OF ZAGREB USING SWISS METHOD SN 640 024

Hrvoje Pilko, B.Eng.

University of Zagreb

Faculty of Transport and Traffic Sciences

Vukelićeva 4, 10000 Zagreb, Croatia

[email protected]

Nikola Božić, B.Eng.

[email protected]

Nikola Šubić, B.Eng.,

Hrvatska osiguravajuća d.d.,

Ivana Lucića 2a, 10000 Zagreb, Croatia

[email protected]

ABSTRACT

In the recent twenty years the roundabouts have become the most frequent form of intersections on the

roads of the European countries. In practice there are many methods for the calculation of the roundabout

capacities, and all are specific and implemented for various types of roundabouts. Capacity calculations require

high-quality field measurements and the collection of necessary data. The paper presents the Swiss method for

calculating the capacity of roundabouts – Method SN 640 024. It has been used to analyze the capacity and

efficiency indicators according to real measurements at 15 roundabouts located in the urban and suburban parts

of the City of Zagreb. The research results should be used, among other things, for the selection method that

would be used in the calculation of the roundabout capacities in Croatia.

Keywords: roundabout, capacity, capacity calculation method

1 INTRODUCTION

In road network the intersections are the most sensitive points at which sufficient

capacity and safety of all the traffic participants need to be provided. Special attention should

be paid to roundabouts, because the laws of traffic flows there are less known. Although the

roundabouts have been implemented in the recent thirty years as the most frequent form of

intersections to calm traffic on the roads of the European countries, the advantages that

influenced greater popularity of constructing these types of intersections, in relation to

classical at-grade intersections, are reflected in their high capacity and traffic safety. Since the

planners and designers first faced the issue of capacity determination and dimensioning, these

gained quite reasonably the priority status. For roundabout capacity modelling a large number

of models from various countries has been developed, and the very selection of the model

requires good field measurements to collect the necessary data [8, 10]. The paper will present

the analysis of the application of the Swiss method SN 640 024 on the actual data collected by

measurements. The necessary data were collected at fifteen roundabouts in the city of Zagreb

with single-lane entry/exit and circular roadway. A more detailed analysis of traffic flows at

the intersection means the assessment of the capacities and adaptability of the observed

solution to the needs of different traffic modes and their users. The indicators that are

measurable and observed in the qualitative assessment of the intersection include: traffic

intensity, degree of saturation flow, capacity reserves, queuing line lengths at the intersection

entry, mean waiting time and the level of service.

mailto:[email protected]





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2 SWISS STANDARD SN 640 024

2.1 Brief description of the procedure

In order to make the applicative part of the work clearer, the scope of application,

methodology of dimensioning and the purpose and limitations of the method procedure have

been presented.

The Standard / Method SN 640 240 is applicable for the single-lane roundabouts at

which there is no weaving, and with traffic-free central island. The Standard describes the

procedure for traffic and technical dimensioning and assessment of the traffic quality at

roundabouts within and outside the towns, and it is applied to each entry-exit part of the leg.

The purpose of the Standard is the selection of the procedures to determine the efficiency,

traffic quality and the loads on the existing roundabouts (supply-oriented procedure), i.e. for

the dimensioning of new roundabouts based on the predicted future needs (demand-oriented

procedure). It also gives instructions to assess the roundabout traffic quality according to the

existing traffic intensity (analysis of efficiency). The Standard must not be applied at

roundabout at which different priority rules are valid (priority to the right, crossing of tram or

train / with or without light signals, and pedestrian traffic across the island), for roundabout

with weaving, for double roundabouts, and for markedly oval forms of roundabouts [4, 9].

2.2 Procedure and criteria for dimensioning

In traffic and technical dimensioning we distinguish three procedures (Figure 1).

Figure 1: Procedures of traffic and technical dimensioning with the most common applications [4]

Procedures are usually applied according to certain criteria i.e. in the following cases:

1. According to supply:

a) Initial state – existing intersection with circular traffic flow;

b) Aim – determining the traffic quantity that can be successfully handled by the

existing roundabout.

2. According to demand:

a) Initial state – expected traffic intensity;

b) Aim – dimensioning of the intersection with circular traffic flow.

3. Efficiency analysis:

a) Initial state – existing roundabout with known traffic intensity;

b) Aim – efficiency balance.

The working steps depend on the selection of the procedure and criterion/case, and they

are presented in Table 1. The efficiency analysis can be carried out in one step only, whereas

multiple iteration of the steps will be required for the supply- or demand-oriented procedures.



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Table 1: Working steps of the three procedures for traffic and technical dimensioning [4, 9]

Procedure According to supply

criterion

According to demand

criterion

According to efficiency

analysis criterion

1st step Checking the functioning

method of the roundabout

"Draft" of the roundabout

and determining the number

of roadways

Checking the functioning

method of the roundabout

2nd

step Determine:

- the relevant traffic intensity / volume of traffic arriving to the intersection at each entry;

- correction regarding combined traffic, and possible longitudinal gradient;

- traffic intensity for the actual situation at all entries and exits, and on the circular roadway in

the area of approaches.

3rd

step Determine at each exit:

- the efficiency / capacity of exit – actual state;

- comparison of traffic intensity at exit: efficiency / capacity at exit

(actual state).

4th

step Determine at each entry:

- efficiency / capacity of entry – actual state;

- traffic quality and respective traffic intensity.

5th

step Comparison at every entry: corresponding traffic intensity; relevant traffic intensity for the

actual state.

6th

step Assessment of operational level.

7th

step Checking at each leg: load, efficiency / capacity of traffic quality / relevant intensity for the

actual state.

8th

step Data on the feasible traffic

intensity (regarding load)

Data on roundabout

dimensioning (indicating

load and measures that need

to be undertaken)

Data on reserve / shortage of

capacity and level of traffic

quality (regarding load)

Iteration of

steps

In case of change of the

actual intensity of traffic /

approach at roundabout in

step 2

In case of measuring the

number of lanes in step 1

(or possibly some other

roundabout design)

No iteration

If in step 3 the traffic intensity at one exit exceeds the actual capacity of that same exit,

the traffic in the circular lane may be blocked completely for a longer period of time. In that

case it is better to select other forms of intersection [9].

2.3 Traffic efficiency and quality

The traffic and technical dimensioning of the roundabouts is usually done based on the

hourly traffic intensity. The relevant traffic intensities are primarily considered to include the

following: average evening traffic peaks in % of average daily traffic, and relevant traffic

calculation from the constant curves. In case of the already existing roundabouts it is

necessary to use the data obtained by traffic counts. However, the traffic intensity can be

assessed also by various mathematical models [6]. Since the roundabouts are very sensitive to

traffic volume oscillations at all their entries, in special cases the traffic intensity in 15-minute

intervals can also be used. This refers first of all to roundabouts in the vicinity of highway

exits, in the vicinity of tunnels or on passenger public transport lines. The traffic intensity

needs to be established for every entry (QEi) and exit (QAi) of the leg, and on the circular

roadway (QKi). If there are pedestrian crossings on the approaches, the number of pedestrians

(FGi) should also be taken into consideration. It is useful to make a load plan as presented in

Figure 2 [9].



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where:

QKi (PCU/h) – traffic volume on circular

roadway at entry level;

QEi (PCU/h) – traffic volume at entry;

QAi (PCU/h ) – traffic volume at exit;

FGi (pedestrian/h) – pedestrians who cross

entry / exit.

Figure 2: Layout example of the load plan which presents clearly the valid traffic volume intensities; QKi,

QEi, QAi, and FGi [9]

The traffic conditions in the reference state correspond to the standard application

scope, and are converted for all the vehicle categories exclusively into equivalent passenger

car units (PCU). Adequate coefficients for the conversion are indicated in Table 2. At the leg

entry, the passenger car unit values depend on the longitudinal gradient, and are also

presented in Table 2 [9].

Table 2: Passenger car unit values regarding incoming traffic and longitudinal gradient of the approach

[9]

Longitudinal

gradient¹)

Vehicle categories

s (%)

Bicycle /

moped

Motor-

cycle

Automobile Truck (and

bus)2)

Truck with

trailer

(and tram)2)

Motor

vehicles3)

+4% - 0.7 1.4 3.0 6.0 1.7

+2% - 0.6 1.2 2.0 3.0 1.4

±0%4)

0.5 0.5 1.0 1.5 2.0 1.1

-2% - 0.4 0.9 1.2 1.5 1.0

-4% - 0.3 0.8 1.0 1.2 0.9 1)

Longitudinal gradient in the direction of entry (+ for climb, - for descent); 2)

If there is no distinction between truck and truck with trailer, the value for truck with trailer has to be used; 3)

On average taking into consideration mixed traffic; 4)

Equivalent values for circular lane, and for approaches without gradient.

The pedestrians can reduce the capacities as well as the quality of traffic at a

roundabout. The influence of pedestrian traffic on the roundabout capacity at the intersection

entry is taken into consideration by using the corrective factor fF from Figure 3. The influence

of the pedestrians on the capacity at the intersection exit has been presented in Figure 4 [9].



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Figure 3: Corrective factor fF for the influence of pedestrian traffic Fgi (pedestrian/h) on single-lane entries

to roundabout [6]

Without the disturbing influences, the single-lane exit capacity from a roundabout

(LAi,MAX) amounts to some 1,200 to 1,400 passenger car units per hour. The influence of

pedestrians who cross the exit leg reduces the exit capacity LAi. This reduction ranges in the

area between the two curves shown in Figure 4 [9].

Figure 4: Approximate assessment of single-lane roundabout capacity regarding the influence of

pedestrian traffic FGi [9]

At all exits it should be checked whether the exit traffic intensity (QAi) is lower than the

capacity of this exit (LAi) for the actual state – QAi ≤ LAi at all exits. If this condition is not

fulfilled at one of the exits, then other solutions need to be found.

The entry capacity (LEi) at a roundabout depends directly on the traffic intensity on the

circular roadway QKi, and is determined for the respective intersections from Figure 5. Here,

the roundabout should be distinguished regarding the number of lanes in the area of entry and

the circular roadway [9].



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Figure 5: Dependence of roundabout entry capacity (LEi) on the intensity of traffic on the circular

roadway (QKi), and on the number of traffic lanes X/Y (X= at entry; Y= on circular roadway) [9]

In case there is pedestrian traffic the capacities determined in this way have to be

multiplied by appropriate correction factor fF. The roundabout is efficient only if the capacity

is not exceeded on any of the entries. The difference between individual capacities and the

existing traffic intensity at the entry is marked as the existing entry capacity reserve (R)

(Figure 6).

The flow quality / level of service is assessed based on the capacity reserve and the

calculated waiting times. The main criterion is the mean waiting time of the motor vehicles

that is calculated for every entry separately. The level of service F is not defined by the mean

waiting time. The border between levels E and F is the roundabout capacity. For the entire

roundabout, the leg with the lowest level of traffic quality is relevant. The mean waiting times

are determined by means of Figure 6 for every entry separately, depending on the capacity

reserve [9]. Table 3: Determining the level of service [9]

Level of

service

- through-flow at intersection

- waiting times / mean waiting time at approach

Traffic

quality

Queuing

(queue length)

A - almost undisturbed

- majority of vehicles without waiting (≈ 5s) / ≤ 15 s

excellent no queues

B - disturbed to a low extent

- acceptable waiting times / ≤ 25 s

good almost no queues

C - frequent influence of motor vehicles

with right-of-way

- waiting times much longer / ≤ 25 s

satisfactory minor queues

D - all motor vehicles are disturbed

- for some motor vehicles partly

longer waiting times/ ≤ 45 s

sufficient sometimes longer

queues

E - constant disturbance with temporary overload

- very long and very dissipated

waiting times / > 45 s

poor partly long queues not

dispersing

F - overload during the entire hour of time (inflow

greater than capacity)

- very long waiting times / no data

completely

unsatisfactory

partially very long

queues not dispersing



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Figure 6: Mean waiting times depending on capacity and capacity reserve [9]

The level of service D is recommended as the basis of dimensioning. Thus, at all entries

there has to be a capacity reserve R ≥ 100 PCU/h. The queue length caused due to standstill

has to be calculated on the basis of the mean waiting time for each entry separately. This

criterion is valid for dimensioning if 90% queue length blocks the adjacent intersections or the

long queues are less than desirable due to safety reasons. In special cases e.g. at roundabouts

in the area of highway connections, the 99% length of the queue has to be calculated. The

95% queue length in standstill is determined from Figure 7, from the dependence on the entry

capacity (LEi), and the level of entry congestion (X= QEi/LEi). The actual queue length is

obtained when for each PCU the length of 6 metres is taken [9].

Figure 7: Presentation of 95% queue length caused by traffic jam in the number of PCUs [9]

The roundabouts with low to medium traffic intensity provide constant driving regimes

with reduced noise and harmful emissions, so that the urban adequacy of the intersection has

to be tested [9].



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3 ANALYSIS OF ROUNDABOUT CAPACITIES IN THE CITY OF ZAGREB

The capacity analysis has been carried out at fifteen roundabouts which are located in

the urban and suburban parts of the City of Zagreb. The selected intersections were classified

according to their size and functions into three groups: Mini RKT1 - RKTm (Dv ≤ 26m), Mini

intersections in the function of small intersections – RKT(m) M, and Small RKT – RKTM

(22m ≤ Dv ≤ 35m) [1, 3]. The design parameters of the analyzed intersections are presented in

Table 4, whereas Table 5 shows all the calculated traffic parameters according to method SN

640 024. The urban adequacy of the intersection regarding noise and harmful emissions has

not been studied due to various legal provisions and regulations.

Table 4: Design parameters of selected intersections [1, 3]

Table 5: Calculations of traffic parameters of selected intersections

1 RKT - roundabout



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3.1 Analysis and comparison of research results

By applying the mentioned method for capacity calculation at fifteen roundabouts the

following results have been obtained which have been presented in Graphs 1, 2 and 3 for the

sake of easier comparison.

Graph 1: Relation of intensity and capacity of entry for each approach of the observed roundabouts



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Graph 2: Relation of intensity and capacity of exits for each approach of the observed roundabouts

From graphs 1, 2 and 3 we can conclude that the observed roundabouts are

undercapacitated, and that the saturation flow levels are within the limit of optimal value X≤1.

However, out of fifteen roundabouts only one intersection is overcapacitated with the degree

of saturation flow greater than 1 (1,035), and thus does not satisfy one of the main conditions

of the Standard (QAi ≤ LAi). Due to the unsatisfactory degree of saturation flow, long waiting

queues are formed at the roundabouts and there are time losses, reflected through vehicle

delays, and the level of roundabout safety is significantly reduced. The influencing factors on

such values are: position and function of the roundabout in the traffic network, design

roundabout elements, traffic lane quality and drivers’ behaviour.

Graph 3: Relation between degrees of saturation flow of entry and exit for each approach of the observed

roundabouts

Since roundabout Petrova-Bukovačka-Prilesje does not satisfy the mentioned condition,

certain parameters are not possible for the calculation. Therefore, the comparison of the levels

of service was carried out for the remaining fourteen intersections (Graph 4). From Graph 4

we can conclude that as much as 86% of the observed roundabouts provide excellent level of

service – A, whereas only 14% provide satisfactory level of service – C.



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Graph 4: Relation of levels of service for 14 roundabouts

4 CONCLUSION

The carried out analysis and comparison of the obtained research results on the

observed roundabouts lead to the conclusion that the capacity and traffic indicators are within

acceptable values. This confirms the thesis that the used foreign method can be applied also in

Croatia for calculating the roundabout capacity with a single lane at entry/exit and on the

circular roadway. This is supported by the fact that the applied method, unlike some other

methods [2, 10], uses also the intensity of the pedestrian traffic which has a significant

influence on the capacity and intersection safety. However, out of the analyzed fifteen

roundabouts in the city of Zagreb, the results of only one are not satisfactory. The calculated

value of the degrees of saturation flow deviates significantly from the limiting value and

places the observed intersection under a big question mark regarding general functionality,

serviceability, throughput capacity, and safety. Furthermore, at the existing roundabouts in

Croatia it is necessary to carry out systematic control of traffic flows, and to undertake further

research regarding capacity by applying the used method and other foreign methods.

LITERATURE

1. Korelacija oblikovnosti i sigurnosti u raskrižjima s kružnim tokom prometa (Correlation

of Design and Safety at Intersections with Circular Traffic Flow - study under way,

2008-2011), Faculty of Transport and Traffic Sciences, Zagreb

2. N. Šubić, H. Pilko, M. Matulin, "Analysis of roundabout capacities in the city of

Zagreb", Conference proceedings Transport, Maritime and Logistics Sciences, 13th

ICTS 2010, Portorož, Slovenia, May 27-28, Faculty of Maritime Sciences and

Transport, 2010

3. Prometna analiza i unapređenje sigurnosti i protočnosti raskrižja s kružnim tokom

prometa, FPZ/PGZ Study, Faculty of Transport and Traffic Sciences, Zagreb, 2009, pp.

34-136

4. N., Božić, "Provjera kapaciteta kružnog raskrižja metodom SN 640 024", Diploma

paper, Faculty of Transport and Traffic Sciences, Zagreb, 2009, pp. 42-88.

5. I., Legac, Raskrižja javnih cesta/Cestovne prometnice II (Public Road Intersections /

Roads II), Faculty of Transport and Traffic Sciences, Zagreb, 2008, pp. 87-120.

6. T. Tollazzi, M. Šraml, T. Lerher, "Roundabout Arm Capacity Determined by

Microsimulation and Discrete Functions Technique", Promet-Traffic&Transportation,

Vol. 20, No.5, 2008, pp. 291-300.

7. Smjernice za projektiranje raskrižja u naseljima sa stajališta sigurnosti prometa

(Guidelines for design of intersections in populated areas from the aspect of traffic

safety) (proposal). Faculty of Transport and Traffic Sciences and HC/PGZ, Zagreb,

2004, pp. 77-84.



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8. P., Mateljak, "Proračun kapaciteta kružnih raskrižja" (Calculation of roundabout

capacities) (workbook), Ülm-Zagreb, 2003, pp.4-105.

9. Vereinigung Schweizerischer Strassenfachleute, Schweizer Norm SN 640 024,

Leistungfähigkeit, Verkehrsqualität, Belastbarkeit, Knoten mit Kreisverkehr, VSS,

Zürich, 1999, pp. 1-20.

10. N. Wu, "Eine universele Formel für Berechnung der Kapazität der

Kreisverkehrsplätzen", Bochum, 1997.

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