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Signalized Turbo Circle; design and performance

Paper 15-2987 for Annual Meeting 2015, Transportation Research Board

Lambertus G.H. Fortuijn

Transport & Planning Department, Faculty of Civil Engineering and Geosciences, TU Delft (Delft University of Technology), Stevinweg 1, 2628 CN Delft, The Netherlands /

Turbo Traffic Solutions, C. Franckrode 19, 2717 BA Zoetermeer, The Netherlands Tel. +31 6 202 805 39

E-mail: L.G.H.Fortuijn@tudelft.nl LGH@Fortuijn.com And

A. Maria Salomons

Transport & Planning Department, Faculty of Civil Engineering and Geosciences, TU Delft (Delft University of Technology), Stevinweg 1, 2628 CN Delft, The Netherlands Tel +31(0)152788556

E-mail: A.M.Salomons@tudelft.nl

Word counting Signalized turbo circle;design and performance

Total counted words incl.References 5545

Title page 134

Number of words exclusive Title Page 5411

Number of figures times 250 8 2000

Total 7411

Limit 7500

Reserve 89

Abstract (in total) 249

Keywords: Traffic circle, Signalized, Turbo circle, Turbo roundabout, Design traffic circle

(2)

ABSTRACT 1

2

A roundabout or traffic circle equipped with leg-by-leg traffic control should not be 3

regarded as a robust solution, because of the accumulation of unnecessary waiting time. 4

Two-phase control is a better alternative. Therefore the design should be adjusted by 5

enlarging and changing the shape of a roundabout. This study considers a signalized 6

traffic circle of a moderate size, with a diameter of 105 m. It will have radially 7

connecting approach legs –to reduce the entry speed– and the departure legs that exit the 8

roundabout tangentially, to create space for queuing left-turn vehicles. The resulting 9

shape is called a Turbo Circle. To tailor a traffic circle expressly to the application of 10

two-phase controlled traffic lights, left-turn and through going directions should be 11

separated. 12

13

In the design of the Turbo Circle, mountable, elevated guide islands are recommended 14

to avoid cut-off turns. 15

16

An analytical model has been developed to compare the cycle lengths of a Turbo Circle 17

and a four-leg crossroads, with the same number of lanes, i.e. two per direction. From 18

this it may be concluded that a Turbo Circle with a diameter of 105 m proves its 19

usefulness in situations with high traffic volumes. Above 6,000 pcu/h, the cycle length 20

of a Turbo Circle tends to be less than half that of a four-leg crossroads, resulting in 21

substantially less delay. The total capacity will thus also be higher, provided that 22

opposing left turn volumes are not more than 130% to 160% of the through-going 23

volumes from the same legs. 24

25 26

(3)

1 INTRODUCTION 27

This paper deals with the problem how to create a signalized traffic circle which can 28

handle a substantial amount of left-turn traffic under the condition of a limited diameter. 29

30

If a roundabout has a small diameter (outer diameter up to approx. 60-70 m), traffic 31

cannot queue on the roundabout itself, because the roundabout segments are too short. 32

In such cases, only the following traffic control methods can be used: 33

- full signalization with leg-by-leg control (also called split phasing); 34

- roundabout metering signals on selected approaches. 35

A simple explorative study shows that roundabouts are too small to be equipped 36

effectively with permanently operating traffic lights That is why metering signals are 37

sometimes used at compact two-lane roundabouts (1). 38

39

To use full signal equipment effectively, it is necessary to enlarge and change the shape 40

of a roundabout. Generally, these enlarged signalized roundabouts are called signalized 41

traffic circles. 42

43

2 TRADITIONAL SIGNALIZED TRAFFIC CIRCLE 44

45

2.1 Features of the traditional traffic circle 46

47

The traditional signalized traffic circle has the following features: 48

- legs that connect tangentially, both in the approach and departure directions; 49

- a large diameter; 50

- lines that can be spiral-shaped, but without consistent branching according to 51

driving direction. 52

53

The tangential connection of the legs on a signalized traffic circle results from the fact 54

that the general traffic flow on the circle was originally based on the weaving of the 55

specific traffic flows on the circle. As soon as these circles were provided with traffic 56

lights, this necessity disappeared. Nevertheless, the tangential form of connection still 57

has its uses on a signalized circle. One positive aspect is the fact that this form creates 58

distance between the departure leg and the approach leg. This distance helps provide 59

positioning room for the traffic turning left, which in turn is necessary for the 60

application of a two-phase control (2). Another positive aspect is the fact that the angle 61

between conflicting parties is small, which reduces the conflict speed (the resulting 62

conflict speed is about 60% of the vehicle speed). However, the speed at a tangentially 63

connecting approach leg can be high; this higher speed can undo the advantage of the 64

smaller conflict angle. 65

66

Another disadvantage of tangentially connecting the approach leg is that the conflict 67

areas are large in comparison with the remaining positioning room, due to the small 68

angle between the roundabout and the connecting legs. To ensure that there is adequate 69

(4)

positioning room on the circle, the traditional signalized traffic circles were usually 70

designed with a large diameter. An external diameter of 125 m is no exception (see 71

Figure 1). Diameters of 200 m also exist. This is the reason why these traffic circles 72

occupy so much space. 73

74

75

Figure 1 Two types of Signalized Traffic Circles:

76

left Roundabout Kooimeer in Alkmaar (a conventional type)

77

and right Hofplein in Rotterdam (an adjusted type) (Google Maps)

78 79

At first, Dutch signalized traffic circles were equipped with concentric lines, called the 80

‘conventional’ marking in the Dutch Guideline for signing and marking roads 1991 (3), 81

(see the two left pictures in Figure 2).The basis for the arrow markings was not formed 82

by the entire circle, but rather continuously by the next partial intersection. According to 83

these markings, the middle entry lane can be used for turning right, turning left, or 84

driving straight ahead. The left entry lane can be used for both left turns and through 85

traffic, while slightly further ahead the lane is reserved for traffic turning left. The 86

consequence of this marking system is, that many weaving movements are necessary at 87

a short distance and that the outer lane on the circle is not used properly. In practice, the 88

conventional lines caused so many problems that they were increasingly replaced by 89

spiral lines  the second stage in the development. In the third stage, finely, the arrows 90

are based on guiding motorists through the entire traffic circle (4), (the third picture in 91

Figure 2). 92

93

94

Figure 2

 

Development in marking of concentric traffic circles

95 96

(5)

The following comments can be made: 97

a) The spiral lines are an improvement over the concentric lines. When the circle 98

diameter is approx. 100 m, however, the traffic on the circle underutilizes the left 99

lane, because the drivers on the circle have to change lanes at a short distance in 100

advance of exiting. This limits the capacity for the traffic turning left. 101

b) To limit the width of the entry points, the two left entry lanes are intended for both 102

turning left and going straight ahead. If left turning traffic is limited, a two phase 103

control is beneficial, as is proved at the Kooimeer roundabout in Alkmaar (Figure 1, 104

left). For this roundabout, a study from 1986 (5) showed that the two-phase control 105

has a capacity that is 23% higher and a cycle time that is 27% shorter than that of a 106

four-phase control. However, if the left turning traffic increases, queues are formed 107

that blocks the traffic flow moving forward on the left through lane, and on the circle 108

the left lane for the left turns is not used properly. In this case four-phase control is 109

necessary. 110

111

2.2 Examples of variant solutions 112

In response to traffic flow problems, traffic circles can also be equipped with lines that 113

make changing lanes unnecessary. For some existing traffic circles, lanes solely 114

intended for flows turning left are already available at the approach. This can result in a 115

significant increase in the capacity of such a circle with a relatively small diameter. An 116

example is the Hofplein in Rotterdam (Figure 1). 117

118

The Dutch literature lacks a systematic discussion of the importance of lane markings 119

on the circle in relation to the possibilities and conflicts with traffic signal control. 120

The study of Ma et al. (6) is based on spreading the left-turn traffic over several lanes on 121

the second segment after entering via one lane on the first segment. But that solution 122

holds the disadvantage of the inefficient use of several lanes in a short distance on a 123

traffic circle with a limited diameter. 124

125

2.3 Conclusions on traditional traffic circles 126

In the past, large signalized traffic circles have been built in many different forms, with 127

various modifications over the years. In summary, the weaknesses of the traditional 128

signalized traffic circle are: 129

- high speeds in the conflict areas; 130

- the need to change lanes on the circle in order to make good use of the left turn 131

lanes; 132

- an inefficient use of the space, which is why a large diameter is required. 133

134

In practice, these problems sometimes result in these circles being modified by: 135

- allocating separate lanes for left turns to the entry point; 136

- assigning lanes exclusively to a single direction (Hofplein in Rotterdam). 137

(6)

It can generally be concluded that the combination of the traffic lights and the necessity 139

of changing lanes between successive decision points makes traffic flow problematic on 140

this type of circle. 141

3 TURBO CIRCLE DESIGN 142

3.1 Turbo circle design objective 143

Given the disadvantages of the traditional signalized traffic circle, an alternative design 144

has been sought which meets three objectives: 145

a) adequate course guidance; 146

b) reduction of the approach speed from the approach legs; 147

c) limiting the required space. 148

Objective a) leads to the rejection of circle designs with a single central point in favor of 149

a turbo design with (generally) four centers and resulting spiral lanes. Objective b) leads 150

to the radial connection of the approach legs to the circle. Objective c) requires the 151

roundabout segments to provide maximum positioning room for traffic turning left, 152

without hindering the through traffic. On the one hand, this requires the intersection 153

areas to be as small as possible. On the other hand, separate lanes for the different 154

directions need to be available. 155

156

In the turbo circle design, these three objectives have been converted into five design 157

features: 158

1. a geometric design that matches the spiral-shaped tracing (without unnecessary 159

bends) of lanes on the circulating roadway; 160

2. approach legs that connect radially; 161

3. departure legs that leave the circle directly after the approach legs; 162

4. separate lanes in each direction; 163

5. mountable roadway guide islands (lane dividers) on the circle between lanes leading 164

in different directions, in order to prevent the bend from being cut off at high speed. 165

These features have been incorporated in steps into various designs. These steps offer a 166

valuable insight into the background of the current turbo circle design. 167

For the considered signalized turbo circles, pedestrian and bicycle traffic is excluded. 168

Including slow traffic (especially when combined with a substantial amount of right 169

turning fast traffic) would increase the cycle time hugely, so these traffic streams are 170

crossing on a different level. 171

172

3.2 First step in turbo circle development 173

First of all, a design has been made which incorporates the first three design features 174

mentioned above. The fourth design feature was initially not used; instead, the design 175

had three lanes on the circle segments, as was customary according to the Dutch 176

guidelines. The difference with this, however, is that the left turn lane on the circle 177

begins directly across from the connecting leg. Accordingly, the approach leg gets a 178

separate positioning lane for left turns (see Figure 3). Design feature 1 will be realized 179

(7)

by using four translation axes, over which the radial centers are staggered, as is the case 180

with the rotor roundabout. In this design, the lanes shift position to the outside at each 181

quadrant of the circle. The approach legs connect radially while the departure legs bend 182

off of the circle directly afterwards. This is shown in Figure 3. This diagram was first 183

published in a contribution to the 'Verkeerskundige Werkdagen' traffic engineering 184 conference in 2000 (7). 185 186 187 Figure 3 Geometric 188 design of approach 189

and departure legs on

190

a modern signalized

191

traffic circle with

192

choice lanes on the

193 circle (significant 194 chance of blockage) 195 196

In this design, changing lanes on the circle is no longer necessary. There are, however, 197

still choice lanes on the circle, on which traffic can choose whether or not to leave the 198

circle. On a roundabout without traffic lights, these lanes, which are used for different 199

directions, offer great benefits - they allow a roundabout to optimally process changing 200

volume patterns. Nevertheless, as noted before, choice lanes have a major disadvantage 201

on a signalized circle: in the place where left turning traffic and through traffic share a 202

lane, cars waiting to turn left at a red light are sure to block the passage of cars that wish 203

to exit the circle. This usually prevents a circle with a small diameter from functioning 204

well. Based on this qualitative analysis of the traffic flow, a quantitative comparison of 205

this design with a four-leg intersection is not further investigated. 206

The fact alone that joint use of a lane by both through traffic and left turning traffic is 207

not a feature of a standard four-leg crossroads, makes generalizations difficult. 208

209

3.3 Basic turbo circle design 210

Given the problems caused by the solution from Figure 3, a turbo circle has been 211

developed that has exclusive lanes for each direction (8). To simplify matters, the basic 212

design features two lanes per direction, see Figure 4. 213

Depending on the volume pattern, the number of lanes per direction can be varied. 214

The features of this design are summarized below: 215

- The entry point connects radially to the circle in order to decrease speed and limit 216

the size of the surface area of the intersection. To ensure the speed reduction for 217

radial entries, shields at eye level are vital to indicate from a great distance that also 218

for through going traffic a turning maneuver is necessary. 219

- The through and left turn lanes are intended exclusively for these directions, as a 220

result of which the feasible queuing length on the circle for left turning traffic is 221

twice as great as it would be with a combination of these directions. 222

(8)

- The lanes for the different directions on the circle are separated by mountable lane 223

dividers which are 30 cm wide. They are already introduced at the entry points. 224

These mountable lane dividers are meant to improve course guidance on the circle 225

and prevent bends from being cut-off, thus helping moderate speed during low 226

traffic periods (as indicated by the radii of the pass through curves). 227

- The curve radii are quite narrow; the inner radius of the circle being discussed here 228

is 30 m. 229

- The width of the roadways gradually decreases. After branching off from the 230

connecting entry point, the width of the inner circle roadway is 10 m. After one 231

segment (a quarter of the circle further) it has become the middle roadway, and its 232

width is decreased to 9 m. One segment further it has decreased to 8 m, and merges 233

into the two-lane exit. These widths have been chosen in order to ensure that two 234

truck combinations next to each other have sufficient room to drive on the circle. 235

- The lanes on the circle have been traced in such a way that the curve radius after the 236

entry curve only increases (and does not alternate between increasing and 237

decreasing). To this end, staggered central points are used in the circle design. 238

- On the side of the roundabout center, the bend width has been expanded outwards (a 239

bend in the direction of the circle's center) in order to provide the leftmost truck with 240

the opportunity to maintain enough distance from a (large) adjacent vehicle when 241

entering the circle. 242

- The driving side of the edge markings lies 45 cm from the roadway edge or the 243

guide strip (lane divider). 244

245

Figure 4 Turbo circle (signalized and with exclusive lanes for each direction)

246 247

3.4 Through-going speed of passenger cars 248

The through-going speed of passenger cars driving straight on a circle is determined by 249

the bend of the driving curve. This bend depends on three factors: 250

 the inner radius of the circle; 251

 the roadway width; 252

(9)

 the positioning of the approach leg with respect to the central point of the circle. 253

The required positioning length determines the diameter of a turbo circle. The road 254

serviceability for trucks determines the lane widths. The through-going speed can then 255

only be limited by the position of the approach leg with respect to the circle; the further 256

to the left, the greater the angular rotation and the smaller the through-going speed 257

becomes. There are limits to how far the approach leg can be shifted to the left, 258

however. If it shifts too far to the left, there is another danger - the direction of the 259

course, left or right, becomes unclear. When both matters were weighed up, the choice 260

was made to position the approach legs in such a way that the extended part of the left 261

'road side' of the through lane runs through the central point of the circle. 262

263

4 TURBO CIRCLES WITH WIDE GUIDE ISLANDS 264

4.1 Marking problems on a compact turbo circle 265

When a circle is constructed according to the description in paragraph 3.3, the course 266

guidance for the through-going movement at the approach to the circle is provided 267

solely by the markings. However, line markings are inadequate for this purpose (see 268

inset Figure 4). This course guidance can only be correctly provided with the help of 269

robust road surface LEDs. However, these were not available when the first turbo 270

circles were being constructed. In order to eliminate the risk of maneuver errors at the 271

approach to the circle, an alternative solution has been found: mountable traffic islands 272

on the circle. See Figure 5. 273

274

4.2 Mountable guide islands 275

There are two reasons why a mountable guide island is preferable to a non-mountable 276

solution: 277

 a non-mountable guide island must be marked by a sign, which itself increases the 278

risk of collision; 279

 a mountable, elevated guide island always allows a driver to rectify an initially 280

wrong lane choice at a low speed, or to deal with an error made by a fellow road 281

user. In terms of Sustainable Safety, this is also called the 'forgiving' aspect of a 282

solution (9). 283

284

The most important aim of the guide islands is to provide better guidance. They do have 285

a disadvantage, however: they make it more difficult to gain a clear overview of the 286

circle. This is especially true for passenger car drivers; truck drivers have a much better 287

overview (10). Due to the multiple functions of the traffic islands (providing guidance, 288

functioning as an 'error correction zone' and providing space for the Traffic Control 289

Installation masts), the first part (aligned with the driving direction) must be mountable, 290

7 cm high and bordered by a rumble strip. This is lower than the guide islands on the 291

turbo circles already constructed, and results in an improved overview of the circle. In 292

the approach direction, the traffic island must be bordered by a higher strip (11 cm), 293

(10)

because that part of the island should draw attention to an obstacle. A traffic sign can 294

then be placed on that part at the level of the stop lines on the entry point. That high part 295

must quickly slope down to 7 cm, because a high traffic island limits the ability of 296

passenger car drivers to gain an overview of the situation. 297

A black and white pillar can be used as an obstacle sign at the level of the stop lines on 298

the circle, as well as on the branching point between the circle and the branching leg (as 299

long as there are obstacles up head). 300

301

Figure 5 Turbo circle with traffic islands (number of lanes for turning right and left

302

varies per entry)

303 304

5 COMPARISON WITH OTHER CROSSROADS SOLUTIONS 305

5.1 Aspects for consideration 306

In order to make an informed choice of a traffic solution in the form of a turbo circle, 307

other possible crossroads solutions need to be considered. The following aspects play a 308

role in such a comparison: 309

a) traffic flow in terms of capacity and loss time; 310

b) safety; 311

c) use of space; 312

d) investment and maintenance costs; 313

e) environmental aspects (noise and emission). 314

315

In order to compare these aspects, which are basically incomparable, an economic 316

estimation can be of use. For aspects a) to d), widely accepted methods for determining 317

a macroeconomic cost-effectiveness ratio are available. In the case of the environmental 318

aspect, the matter is more complicated, both with regard to the economic estimation of 319

(11)

it, and to the specific environmental factors that play a role. This article will restrict 320

itself to the two aspects mentioned first. 321

5.2 Traffic flow 322

Below, the traffic performance of a turbo circle is compared to that of a signalized 323

crossroads (11). The same number of lanes was used for both the four-leg crossroads 324

and the turbo circle, namely two for each direction. Traffic movements are labeled as 325

shown in Figure 6. The moment the cycle length reaches the limit of 120 seconds has 326

been defined as a capacity indicative of the four-leg crossroads. In principle, this also 327

applies to the turbo circle, but with an increasing amount of traffic turning left, the 328

positioning length on the circle is the deciding factor for a shorter cycle length and 329

therefore also for the capacity. 330

331

For reasons of clarity, the intensities of all directions have been made dependent on the 332

through-going direction of the leg with the highest volume, in direction 02, by means of 333

four factors: 334

 factor (a) ratio of direction 08 and direction 02; 335

 factor (b) ratio of direction 05/11 and 02 (the side directions evidently have an equal 336

volume); 337

 factor (c) ratio of right turn and through-going – this applies to each approach leg; 338

 factor (d) ratio of left turn and through-going – this applies to each approach leg. 339   10  11 12  01  02  03  04  05  06  07  08  09  71  72  63  62  66  65  69  68  06 0504 10 1112 01 02 03 09 08 07   10  11 12  01  02  03  04  05  06  07  08  09  71  72  63  62  66  65  69  68  06 0504 10 1112 01 02 03 09 08 07   10  11 12  01  02  03  04  05  06  07  08  09  71  72  63  62  66  65  69  68    10  11 12  01  02  03  04  05  06  07  08  09  71  72  63  62  66  65  69  68  06 0504 06 0504 06 0504 10 1112 10 1112 01 02 03 01 02 03 09 08 07 09 08 07 340

Figure 6 Basic configurations of the four-leg crossroads and turbo circle used in

341

the performance comparison, with direction numbering

342 343

As expected, factor c) 'right turn and through-going' has little influence on the capacity 344

comparison. On the other hand, factor d) is a strong determining factor for both the 345

functioning of a four-leg crossroads and that of a turbo circle. Eight variants, 346

represented by the letters A to H, were chosen for the combination of factors (a) and (b), 347

while factor (d) varies from 0.2 to 2. In the calculation process, the volumes have been 348

increased in steps until the criteria for the maximum cycle length (120 s) has been 349

reached or until the positioning room on the circle has been filled up. In this way, 80 350

(12)

variants have been calculated (see (2), Appendix G.4). Capacity calculations follow an 351

analytical model, described (11). 352

Figure 7 shows the development of the total capacity and that of the main flow for a 353

single combination of the factors a) and b) while varying the ratio of left turn and 354

through-going (for both the four-leg crossroads and the turbo circle. The cycle length 355

that elapses when traffic increases is shown for the same variant with an equal amount 356

of left turning and through traffic (Figure 7, above right). 357

358

Based on these calculations, three conclusions can be drawn: 359

 As long as the amount of left turning traffic is smaller than 130% of the through 360

traffic, the capacity of the turbo circle with a diameter of approx. 100 m is greater 361

than that of a four-leg crossroads in all cases. In the volume combination shown, this 362

is applicable as long as the factor d) 'left turn versus through-going' is smaller than 363

1.6. In the case of an evenly balanced volume pattern, however, that turning point 364

only occurs when the amount of left turning traffic is twice as large. 365

 The cycle length, at which a higher capacity is achieved on a turbo circle, is always 366

shorter than that of the four-leg crossroads. As the ratio of left turning traffic 367

approaches the above-mentioned limit, the gain in cycle length increases. In the 368

example of Figure 7, the capacity of the approx. 8800 pcu/h on the turbo circle is 369

reached at a cycle length of 66 seconds, while at a cycle length of 120 seconds the 370

capacity of the four-leg crossroads is approx. 7300 pcu/h. 371

372

Figure 7 Performance of a crossroads versus a turbo circle

373

 The cycle length of a circle is especially favorable in the case of high volumes. In 374

the graph of Figure 7, a circle with a volume lower than approx. 5500 pcu/h does 375

(13)

yield a gain in cycle length, but the difference is minimal. The gain in cycle length 376

of a circle is only substantial in the area above 5500 pcu/h (factor > 2). 377

378

It must be noted, that in this analytical model the stochastic character of arrival patterns, 379

as well as the efficiency loss due to green time extension at sub-saturated flow, are 380

ignored. In reality the capacity will often be lower than determined in the analytical 381

model. 382

383

In the TNO study titled Evaluatie geregelde turbopleinen (10), traffic flow is also 384

discussed. The turbo circle is compared to a four-leg intersection, which is somewhat 385

similar to the Doenkadeplein in terms of traffic volumes and lane arrangement. It has 386

been determined that the cycle time on the turbo circle is 40-45 seconds shorter (–47%), 387

the average waiting time is 7 seconds shorter (–41%) and the maximum queue on the 388

circle is shorter by 6 vehicles (–39%). These findings correspond well to a difference in 389

cycle time, which can be greater than a factor 2 in the case of high volumes. 390

6 TURBO CIRCLE AND TRAFFIC SAFETY 391

The accidents in the before and after periods of only a single turbo circle, the 392

Doenkadeplein in Rotterdam (see Figure 8), can be compared. In the before situation, 393

this was a T-connection of the N471 onto the N209, regulated by traffic lights. After the 394

lengthening of the N471, it became a turbo circle with four connecting 395

legs. 396

397

Figure 8 Doenkadeplein; Turbo Circle with Overhead Signs

398 399

Despite the addition of a leg (and the increase in the amount of traffic), the number of 400

registered accidents per year is smaller after the construction of the turbo circle: in six 401

(14)

year before-period 2.67 per year and in three year after-period 2 per year (level of 402

significance 55%). The pattern has also been distinguished according to the seriousness 403

of the accident, in accordance with expectations: accidents with injuries in the before-404

period 0.83 per year and in the after period 0.33 per year (level of significance 39%); 405

serious accidents: in the before-period 0.33 per year and in the after period 0.00 per 406

year (level of significance 32%). Due to the low figures, however, the differences are

407

not significant, which means that no hard conclusions can be drawn about the safety of 408

the turbo circle. 409

410

For the sake of completeness, the accidents on the Tolhekplein have also been taken 411

into account. No figures exist for the prior situation, while the intensity is 62% of the 412

intensity on the Doenkadeplein. For this reason, these accidents have been multiplied by 413

a factor of 1.6 in order to include them as an imaginary extension to the after period of 414

the Doenkadeplein. This leads to the conclusion that the number of serious accidents 415

decreases with a reliability level of 68% due to the construction of a turbo circle (not 416

significant; in seven years of observation, not a single accident resulting in serious 417

injury has been recorded). 418

419

Outside the Netherlands the safety of the mountable raised lane dividers may be 420

questioned. It may be clear, that registered accidents on 16 spot-years do not indicate 421

that mountable raised lane dividers will have a negative effect on safety. Even more, a 422

large reduction of accidents associated with injury is observed for turbo roundabouts 423

(12). 424

425

In any case, these figures do not indicate that a turbo circle is unsafe, as behavioral 426

scientists expected it would be on the basis of recorded maneuver errors (10). It seems 427

that by labelling 'maneuver errors' as 'unsafe behavior', behavioral scientists – 428

considering the suggestion that this poses with regard to the level of safety of a traffic 429

solution, expressed in numbers of accident victims – have not fully taken into account 430

the speed of the potential conflict partners. The recommendation to be 'cautious in the 431

application of this type of intersection' is not supported by the figures. A higher score in 432

a perception survey does not guarantee greater safety. 433

434

7 CONCLUSIONS AND DISCUSSION 435

In this article, the possibilities of combining traffic lights with roundabouts and/or 436

circles have been studied. It can generally be concluded that traffic lights are only useful 437

on entry points with more than one lane. Even on two-lane roundabouts with a small 438

diameter, however, a complete traffic control barely improves the capacity. There are 439

possibilities for substantially improving the traffic flow with the help of roundabout 440

metering lights. 441

442

On circles with more than two lanes, traffic lights are required for satisfactory traffic 443

flow. Historical research has shown that two-phase control makes better use of the 444

geometric features of a circle for smooth traffic flow than four-phase control. 445

(15)

Two-phase control requires sufficient positioning room for traffic turning left. The 447

traditional traffic circles with tangential connecting approach and departure legs are 448

inadequate for two reasons: the intersection areas require a relatively large area, so that 449

not much positioning room remains, while a traffic flow turning left often blocks the 450

through traffic due to the lane arrangement on the circle. As an alternative to this, the 451

first author designed the turbo circle, which features the following elements: 452

 approach legs connect radially; 453

 departure legs leave the circle tangentially directly following the approach legs; 454

 lanes are movement specific. 455

456

A critical success factor for the use of the turbo circle is the degree to which drivers are 457

able to choose the correct lane on approaching the circle. Guide signs on overhead sign 458

gantries, which feature arrows based on the roundabout symbol, provide adequate 459

information for this purpose (see Figure 8; in the picture the second overhead sign is 460

placed accordance the Dutch Guideline for Directional Signing; the first overhead 461

directional sing has to be placed at the start of the sort section, introduced by a pre-462

directional sign on the road stretch before). 463

464

The turbo circle's advantage in terms of traffic flow –short cycle lengths which result in 465

short queues– is a disadvantage in terms of ease of use; the positioning areas can be too 466

small to provide room for the two consecutive gantries with signs, which are needed to 467

provide a sufficiently clear traffic situation. 468

In order to prepare drivers in advance for the selection of the correct auxiliary lane 469

before they actually reach the lane selection areas, a new advance direction sign was 470

created. A limited study was conducted in order to test various alternative types for 471

effectiveness. On two-lane approach legs, road arrows (in the form of roundabout 472

arrows), over a length of 180 m before the start of the actual lane selection areas, can 473

aid lane selection. 474

In comparison with a crossroads with the same number of approach lanes, an 475

improvement of the total capacity by 1500 pcu/h (from 7000 pcu/h to 8500 pcu/h), 476

combined with a reduction of the cycle length from 120 to 60-80 seconds, is an example 477

of expecting performance. 478

479

The crash history of turbo circles so far shows a pattern of improved safety. However, 480

due to the small number of observation years, no reliable statement can be made about 481

improved safety based on the decreased number of injury accidents following the 482

construction of a turbo circle. 483

8 REFERENCES 484

(1) Fortuijn, (Bertus) L.G.H. (2014) Robustness of Roundabout Metering Systems 485

(RMS) Contribution to the TRB 2014 International Roundabout Conference, 486

April 16-18, 2014, Seattle, WA, USA. Website: 487

http://teachamerica.com/RAB14/RAB14papers/RAB14ppr009_Fortuijn.pdf

(16)

(2) Fortuijn, (Bertus) L.G.H. (2013), Turborotonde en turboplein: ontwerp, capaciteit 489

en veiligheid, T2013/1, January 2013, TRAIL Thesis Series, the Netherlands

490

(Dutch). 491

(3) V&W (1991) Richtlijnen voor de bebakening en markering van wegen. Ministerie

492

van Verkeer en Waterstaat, Directoraat-Generaal Rijkswaterstaat, Den Haag, 493

december 1991. 494

(4) CROW (2005) Richtlijnen voor de bebakening en markering van wegen, 495

Publicatie 207. CROW, Ede. 496

(5) BGC (1987) Evaluatie verkeerslichtenregelingen rotondes, Rijkswaterstaat dienst 497

Verkeerskunde. Bureau Goudappel Coffeng b.v., Kenmerk RWE/593/13/Bn,

498

Deventer, 1987. 499

(6) Ma, Wanjing, Yue Liu, Larry Head, Xiaoguang Yang (2013), Integrated 500

optimization of lane markings and timings for signalized roundabouts, 501

Transportation Research Part C 36 (2013) 307–323.

502

(7) Fortuijn, L.G.H. en P.J. Carton, (2000) Turbopleinen: een beproefd concept in een 503

nieuw jasje, Verkeerskundige werkdagen 2000. CROW, Ede. (Turbo circuits: 504

A well-tried concept in a new guise; no longer accessible on internet).

505

(8) Fortuijn, L.G.H. (2003) Turbopleinen met verkeerslichten; het geometrisch 506

ontwerp. Verkeerskundige werkdagen 2003, CROW, Ede. 507

(9) SWOV (2005) Door met Duurzaam Veilig. Eindredactie Fred Wegman en Letty 508

Aarts. Stichting Wetenschappelijk Onderzoek Verkeersveiligheid SWOV, 509

Leidschendam, 2005. Factsheet: Background of the five Sustainable Safety 510 principles 511 http://www.swov.nl/rapport/Factsheets/UK/FS_Sustainable_Safety_principles.pdf 512 (referred to on 11-11-2014). 513

(10) Horst, A.R.A. van der, M.C.L. Groenewoud, en M.H. Martens, E.C.J. Franx 514

(2008), Evaluatie geregelde turbopleinen. TNO-rapport TNO-DV 2008 C006, 515

TNO, Soesterberg. 516

(11) Fortuijn, L.G.H. en A.M. de Leeuw (2003) Turbopleinen met verkeerslichten; 517

verkeersregeling en –afwikkeling, Verkeerskundige werkdagen 2003, CROW, 518

Ede. 519

(12) Fortuijn, Lambertus G.H. (2009) Turbo Roundabouts: Design Principles and 520

Safety Performance, Transportation Research Record, Journal of the 521

Transportation Research Board, No. 2096, Washington, D.C., U.S.A., 2009,

522

pp.16-24. 523

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