Roundabout design guidelines: Case study of Croatia

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Transport Infrastructure and Systems – Dell’Acqua & Wegman (Eds) © 2017 Taylor & Francis Group, London, ISBN 978-1-138-03009-1

Roundabout design guidelines: Case study of Croatia H. Pilko Department of Road Transport, Faculty of Transport and Traffic Science, University of Zagreb, Zagreb, Croatia

ABSTRACT: The popularity of roundabout application around the world is evident. Due to the inexperience of construction companies and the lack of proper national guidelines, distinctiveness in design is noticeable. In some intersections this led to reduction of Traffic (operational) Efficiency (TE). The purpose of this paper is to analyze: 1) the current state of roundabouts in Croatia; (2) known approaches to using geometry elements of roundabouts to predict TE; (3) overview and comparison of selected design guidelines; and (4) to present and comment the latest Croatian Roundabout Design Guidelines on State Roads 2014 and show examples of good practice. Research results will serve to disseminate the knowledge for proper application and implementation of national roundabouts in order to compare it with international design practice and standards. 1

INTRODUCTION

It also gives a brief overview of the literature on roundabout design and TE in Croatia. Section 3 gives a brief overview and comparison of European, American and Croatian guidelines. In Section 4 recent Croatian roundabout geometry design practice is shown. Section 5 reports the conclusions.

The popularity of roundabouts around the world has driven substantial efforts to optimize their planning and modeling. Analyses of how geometric elements, traffic flow movements, driver behavior and other factors influence roundabout Traffic (operational) Efficiency (TE) have led to the development of numerous simulation-based computational models. In addition, they use primarily empirical and gap-acceptance mathematical models to determine TE (mainly focused on capacity and delay). Croatia has slightly more than 200 roundabouts, of which more than 60% lie within or on the edge of urban areas, and many of them deviate substantially from international standards for roundabout planning, design and modeling, which compromises their TE and traffic safety (Pilko 2014). These deviations are due primarily to the inexperience of construction companies and the use of outdated design guidelines. The country lacks a system for monitoring and analyzing TE, and other parameters, though the government has called for the building and reconstruction of roundabouts as part of its National Traffic Safety Plan 2011–2020 (Pilko 2014). The present research aims to describe and analyze (1) the current state of roundabouts in Croatia; (2) known approaches to using geometry elements of roundabouts to predict TE; (3) overview and comparison of selected design guidelines; and (4) to present and comment the latest Croatian Roundabout Design Guidelines on State Roads 2014 and show examples of good practice. The reminder of the paper is organized as follows. Section 2 contains the literature review on roundabout geometry and TE.

2

BACKGROUND

2.1 Roundabout geometry and efficiency Numerous models for determining roundabout capacity under mixed-traffic conditions suggest that it is strongly affected by geometric elements (Dahl & Lee 2012). Studies of roundabouts in various countries, particularly of single-lane roundabouts in urban areas, have shown that proper design and modeling can significantly improve Traffic (operational) Efficiency (TE) (Vasconcelos et al. 2013; Mauro & Cattani 2012). The most important geometric elements influencing TE (i.e. entry capacity) are entry width, entry radius, flare length, entry angle, Inscribed Circle Diameter (ICD) and number of entry lanes (Kimber 1980; Al-Omari et al. 2004; Dahl & Lee 2012; Yap et al. 2013; Barić et al. 2016). Geometry considerations have also proven useful for analyzing driver behavior in roundabout situations and implications for traffic safety (Muffert et al., 2013; Wang et al., 2002). Methods for predicting traffic accidents have been described based on geometric elements (Maycock & Hall 1984), sight distance (Turner et al., 2009; Zirkel et al., 2013), as well as traffic dynamics and driver behavior when passing through the „potential conflict“ zone of the intersection (Mauro & Cattani, 2004).

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between vehicle trajectory design speeds through the roundabout and observed vehicle speeds.

Studies of roundabout TE, conducted primarily in Western Europe and Australia, have led to several computational mathematical models that have been integrated into various roundabout software engineering simulation tools (e.g. ARCADY and PTV VISSIM). These mathematical models can be classified as (1) empirical, (2) gap acceptance, and (3) microsimulation. Each category has its disadvantages (Yap et al. 2013). Generally countries with updated roundabout design guidelines apply Highway Capacity Manual (HCM) models for analyzing roundabout capacity (Chodur 2005). These models take into account empirical and/or gap-acceptance models, but they do not address the Level of Service (LOS). The capacity formulation for urban single-lane roundabouts developed in HCM is based on the mathematical formulation of geometry parameters developed by Kimber (1980) and formulation of queue length developed by Ning Wu (Wu 2001). Differences among these models in how data are collected and analyzed, as well as deviations between predicted and actual driver behavior, make it difficult to identify the most suitable ones for given conditions (Mauro 2010). Planners and designers should be aware of the specific limitations of these models and the selected model(s) must be calibrated against field data or other validated models to ensure accuracy. Unfortunately, this calibration step is often neglected. In situations where the model is not even developed, it may be advisable to analyze roundabout TE using various approaches, and HCM models may be the most appropriate for such work (Mauro, 2010).

3 3.1

OVERVIEW AND COMPARISON OF DESIGN GUIDELINES International design guidelines

Worldwide positive experiences with modern single-lane roundabouts contributed to further research, development and implementation of other types of roundabouts. Substantial practical experiences initiated the formation of roundabout design guidelines. The recent achievements in this field for European countries like Austria, Germany, Switzerland, Poland, UK and the American are presented here in a brief overview. In general, types of roundabout are defined by spatial limitation, location and traffic capacity. Mini roundabouts are characterized by a small external diameter and traversable central island for large vehicles. They are commonly used in urban environments with average operating speeds of 40 km/h or less. Single-lane roundabouts represent a standard solution and they are characterized by single entry lane, exist lane and circulatory lane. There is a non-traversable central island and they are used both in urban and rural environments. Multi-lane roundabouts have two or more entry/exit and circulatory lanes. Due to a possibility of path ovelap at the entry and the exit as well as higher speeds these types of roundabouts are less safe in comparison with mini and single-lane roundabouts. According to Austrian guidelines (Osterreichische Forschungsgemeinschaft Strasse und Verkehr 2010) the geometry elements in mini roundabouts are determined on the basis of curve of the course of design vehicles. It is recommended that single-lane roundabouts have the external diameter r 26 m (35–40 m) and that multi-lane roundabouts have the external diameter r 40 m (50 to 60 m). German guidelines (Forschungsgesellschaft fur Strassen und Verkherswesen 2006) have determined that mini roundabouts have a diameter of 13 to 22 m and capacity of 18000 veh/day. A central island is traversable and has truck apron raised by 4 to 5 cm. The width of circulatory lane is beween 4 and 6 m with transversal inclination of 2.5% outwards. The entry/exit radii is from 8 to 10 m. Small single-lane roundabouts have the capacity of 25000 veh/day. The external diameter is from 26 to 45 m, the entry radius is from 10 to 16 m and the exit radius is from 12 to 18 m. These dimensions provide great traffic safety. Circulatory lane being 6.5 to 9 m wide consists of driving and traversable part used by large vehicles. Small double-lane roundabouts have one or two lane entry with the entry radii of 12 to16 m,

2.2 Roundabouts in Croatia The in situ work by Legac et al. (2008) on 30 roundabouts in Zagreb examined relationships between main geometric elements and the numbers and types of traffic accidents, traffic flow demand, and numerous other determinants of capacity. That study confirmed that unsignalized roundabouts in Croatia are safer than classical intersections and it showed that geometric elements strongly influence safety. Other authors have focused on applying roundabout models from outside Croatia to the Croatian situation. Their results suggest that imported models can work well, as long as they are calibrated for local conditions (Otković Ištoka 2008; Ištoka Otković & Dadić 2009; Šubić et al. 2012). For example, Ištoka Otković et al. (2013) used neural networks to calibrate a traffic microsimulation model for two urban single-lane roundabouts in Osijek. Šurdonja et al. (2013) optimized geometric elements such as inscribed circle radii, entry/exit radii, entry/ exit approach width, and vehicle path trajectory, while Pilko et al. (2014) examined the relationship

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Urban multiple (two)-lane roundabouts are supposed to have external diameter from 37.5 m to over 55 m and from 40 m to 65 m for rural areas. Single-lane roundabouts are the solutions which, in Polish conditions, ensure a very high level of safety to the users and high capacity. Furthermore, two-lane roundabouts are not entirely optimal solutions since substantial number of drivers use mainly the right lane, both at the approach entry and on the circulatory roadway (Macioszek 2013). The UK guidelines (UK Goverment 2007) not only define geometry elements for mini, single and multi-lane roundabouts, but they also give instructions to: traffic safety, road users specific requirements, the assessment procedure, and conspicuity. The main geometry parameters for mini, single and multi-lane roundabouts are shown in Table 2.

depending on the traffic load and one lane exit with the exit radii of 12 to 18 m. In the circulating area there are two traffic lanes with the total width of 8 to 10 m, but they are not marked with the horizontal signalization. Diameter varies from 40 to 60 m with the maximum capacity of 32000 veh/day. Their diameter is more than 60 m and they have two or more lanes in entry, exit and circulatory lanes (Forschungsgesellschaft fur Strassen und Verkherswesen 2006). According to the German guideliness the designing of small double-lane roundabouts is not allowed due to the reduced safety. The solution of the problem of increased through traffic is found in the use of roundabouts with the spiral traffic course, the turbo roundabouts. Swiss guidelines (Vereinigung Schweizerischer Strassenfachleute (VSS) 1999) have determined that mini roundabouts and single-lane roundabouts are designed with the entry radii of 10 to 12 m, while the approach radius is five times larger. In a properly designed entry the entry angle A has to be as large as possible. The exit radius is from 12 to 14 m. Mini roundabouts with circulatory lane width of 7 to 8 m are characterized by external diameter of 14 to 16 m. Single-lane roundabouts have external diameter of 26 to 40 m. USA guidelines (NCHRP-National Cooperative Highway Research Program 2010) have determined that splitter islands can be raised, traversable or only marked and designed according to design vehicle. The width of circulatory lane in singlelane roundabouts varies from 4.8 to 6 m. Circular shape of a central island is recommanded, but oval, irregular or raindrop shapes can also be used. The entry radius is from 15 to 30 m and the exit radius is from 15 to 60 m. The traversable portion of a central island is 50 to 75 mm raised. Multilane roundabouts have at least one entry with two or more lanes which requires a wider roadway in circulating part of the intersection so that at least two vehicles can travel side by side. The width of circulating double lane is from 8.5 to 9.8 m and of circulating triple lane is from 12.8 to 14.6 m. Firstly an entry is designed with a smaller radius of 20 to 35 m, and then with a radius of 45 m and more. The entry lane can be moved to the left in order to obtain increased deflection which reduces it at exit. Radius of the fastest path is between 53 and 84 m which results in design speed of 40 to 50 km/h. According to Polish guidelines (Generalna Dyrekcja Dróg Krajowych i Autostrad 2004) the geometry elements have determined that mini roundabouts have diameter of 14 to 22 m (exceptionally 25 m), fully traversable central island, entry/exit radii of 6 to 12 m and capacity of 15000– 17000 veh/day. Urban single-lane roundabouts are designed with the external diameter ranging from 14 to 45 m and from 30 to 50 m for rural areas.

3.2 Croatian guidelines The procedures of planning and designing the roundabouts in the Republic of Croatia are based on the current national (Institut prometa i veza 2002) and applied foreign guidelines, especially German (Forschungsgesellschaft fur Strassen und Verkherswesen 2006), Austrian (Osterreichische Forschungsgemeinschaft Strasse und Verkehr 2010) and Switzerland (Vereinigung Schweizerischer Strassenfachleute 1999), positive examples of world practice and empirical practice of designers (especially from the Netherlands). One of the first steps in creating national regulations for roundabout designing were the Guidelines for the Design of Circular Intersection (Guidelines 2002) from 2002 (Institut prometa i veza 2002). The main objective of these guidelines was the standardization of design and implementation of roundabouts on public roads in the country. Then, in the absence of local regulations, designers applied the aforementioned foreign regulations and practices, which resulted in a large number of non-compliant roundabout design. Also, insufficient attention has been made to the criteria of TE. Roundabout Guidelines for State Roads (Guidelines 2014) from 2014 (Deluka-Tibljaš et al. 2014) represent a logical step on the path of creating national regulations for planning and designing of all roundabout types, especially in urban areas. They also represent a significant upgrade of Guidelines 2002 in terms of geometry design elements, TE criteria, defining, planning and implementation of turbo roundabouts, and the importance and necessity for trajectory checking of the relevant design vehicle in the design phase. However, the guidelines do not indicate what models or simulation software should be used for analyzing TE. The Guidelines2014 only briefly presents possible methods and their pros and cons of

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3.3

Austrian (Osterreichische Forschungsgemeinschaft Strasse und Verkehr 2010), British linear regression, Australian (by National Association of Road authorities) and German (Wu 2001) method that can be used for analyzing TE parameters. As the process of roundabout designing is very complex and specific, it is not always possible to apply the optimal geometry design elements and meet all requirements. However, planning and designing in accordance with Guidelines2014 enables unification at the national level, which derives complete and optimal solutions. By making the final legislative framework for designing roundabouts, it is necessary to gradually adopt and implement certified foreign methodologies and practices, and to introduce them to state regulation, with obligatory consideration of all national features. Table 1.a.

Guideline comparison

From the brief overview (Table 1.a.b and Table 2) we can conclude that all European guidelines are more less similar in terms of main geometry design elements. Only the UK geometry parameters are slightly different than the other European. This is also evident when we compare the concepts and dimension with the latest Croatian guidelines. The most evident difference is approximate capacity. The USA guidelines have bigger values of geometry design when compared to European and Croatian guidelines. This is because of longer design vehicle and dimensions related to specific national conditions and driver behavior.

Guideline comparison of roundabout type and main geometry design elements. Roundabout type Mini

Design guidelines/Design Environment External diameter Entry lane (m) Exit lane (m) Circulatory lane Central island Entry radii (m) Exit radii (m) Circulatory lane width (m) Maximum recommended entry design speed (km/h) Capacity (veh/day)

Table 1.b.

A1 26

– – –

Single-lane D2

CH3 USA urban 13–22 14–16 13–27 1 1 1 traversable 8–10 10–12 15–20 8–10 12–14 b30 4–6 7–8 4–10 25–30

CRO4

18000

b15000

10000

A1

CH3 USA urban or rural 26–45 26–40 27–55 r 26 1 1 1 non-traversable 10–16 10–12 15–30 12–25 12–18 12–14 30–60 6.5–9.0 4.8–6.1 30–40

13–25

10–12 12–14 4.5–5.0

D2

25000

CRO4 30–40

10–12 12–14 5.5–7.0

20000

Guideline comparison of roundabout type and main geometry design elements. Roundabout type Multi-lane

Design guidelines/Design Environment External diameter Entry lane (m) Exit lane (m) Circulatory lane Central island Entry radii (m) Exit radii (m) Circulatory lane width (m) Maximum recommended entry design speed (km/h) Capacity (veh/day)

A1

D2

r 40

40–60

10–16 12–25 8–10

30000

12–16 12–18 8–10 50–60 32000

1 – Austrian, 2 – German, 3 – Swiss, 4 – Croatian.

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CH3 urban or rural r 60 r1 r1 r2 non-traversable

USA

CRO4

46–91

50–90

20 r 20 4.3–14.6

10–14 12–16 5.5–7.7 40–50

b45000

r 250000

Table 2.

Guideline comparison of roundabout type and main geometry design elements. Roundabout type Mini

Design guidelines/Design Environment External diameter

PL1

UK2 urban

14–22(25) Entry lane (m) Exit lane (m) Circulatory lane Central island Entry radii (m) Exit radii (m) Circulatory lane width (m) Maximum recommended entry design speed (km/h) Capacity (veh/day)

Single-lane

6–12 6–12 5.0

28 1 1 1 traversable 6–20 8–10 6.0 25–30

15000–17000

18000

PL1

Multi-lane

PL1 urban or rural 22–45* r 37.5–55* 30–50** 28 r 40–65** r1 r1 r2 non-traversable 10–15 10(20) 12–15 12–15 15–20 14–17.5 5.5–8.75 9.1–10 15 30–40 40–50 20000–25000

UK2

UK2

100

12–16 20–100 8–10 40

40000

1 – Polish, 2 – United Kingdom, *urban areas, **rural areas.

4

CROATIAN EXAMPLES

Given the above in section 2.2 and 3.2, the following will give a brief overview of recent implemented and planned forms of urban roundabouts. The presented examples of design practice are designed in accordance with the Guidelines2002, Guidelines2014, and according to aforementioned foreign regulations. Mounted roundabouts are also presented in the latest guidelines, and the first examples of its usage can be seen in the City of Zagreb. Figure 1 shows the latest implementation of mounted roundabout in urban area. Reconstruction of State Road S517 on a road section through the city of Belišće is performed using two roundabouts (Figure 2). The reconstruction eliminates existing dangerous small turn radius and adverse horizontal elements. External diameter of Roundabout 1. (to the right in Figure 3) is 38.00 m, and is the busiest and most important intersection in the city. Roundabout 2. (to the left in Figure 3) has external diameter of 36.00 m. The circulatory roadway width is 6.6 meters, with an additional truck apron width of 1.8 m. The entry/exit radii were designed using vehicle design trajectories for a truck with a trailer length of 18.00 m (Pilko et al. 2015). The first turbo roundabout in Croatia was carried out in the city of Osijek in 2014. (Figure 2). Intersection is designed in accordance with the Guidelines2002 as well as the empirical experiences with turbo roundabouts from Netherland. The most important design elements are: external diameter is 5.15 m (outer space between shifted centers

Figure 1. Roundabout Vončinina—Voćarska, City of Zagreb, Croatia—in traffic 2016.

Figure 2. Turbo roundabout in the City of Osijek, Croatia—in traffic 2014.

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Figure 3.

Roundabouts at State road S517 in city of Belišće, Croatia.

implementation of the existing or development of a new method and its application and validation for Croatia. However, planning and designing in accordance with Guidelines2014 enables unification at the national level, which derives complete and optimal solutions. By developing the final legislative framework for designing roundabouts, it is necessary to conduct a comprehensive and systematic field-analytical research on existing roundabouts. In addition, current national studies may serve as a certain database, as well as a source of empirical knowledge. It is necessary to involve all stakeholders and international experts from this field. Results of this paper can be used for dissemination of the knowledge for proper application and implementation of national roundabouts in order to compare it with international design practice and standards.

of circular segments); inscribed central diameter is 4.95 m (inner space between shifted centers of circular segments); R1  15 m (radius of the inner edge of the road surface); R2  20 m (radius of the outer edge of the outer lane); R3  20.3 m (radius of the inner edge of the outer lane); R4  25.2 m (radius of the outer edge of the road surface). Cycling lanes and tram lines are also designed making this intersection more complex for all road users. 5

CONCLUSION

There is an obvious increase in the application and design of all types of roundabouts in urban areas worldwide. From the brief overview of the selected West European and USA roundabout guidelines we can conclude that West European guidelines are more less similar in terms of main geometry design elements. This is also evident when we compare the concepts and dimension with the latest Croatian guidelines. The most evident difference is approximate capacity. The USA guidelines have bigger values of geometry design when compared to West European and Croatian guidelines. This is because of longer design vehicle and dimensions related to specific national conditions and driver behavior. Due to the lack of Croatian regulations, some studies and designers warn of the distinctiveness of performance, which is at some roundabouts resulted in reduction of TE. Guidelines2014 enables better preparation of national legislation for planning and design of all types of roundabouts. However, the guidelines do not indicate what models or simulation software should be used for analyzing TE. Future studies should make comparative comparison of capacity formulations (TE—Measure of Effectiveness (MOE) of presented guidelines by incorporating real traffic data collected from roundabouts in Croatia. This would enable possible

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Roundabout design guidelines: Case study of Croatia

Transport Infrastructure and Systems – Dell’Acqua & Wegman (Eds) © 2017 Taylor & Francis Group, London, ISBN 978-1-138-03009-1 Roundabout design guid...

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