Roundabout Design at Salh Intersection - A case study · Roundabout is designed by following IRC:...

Roundabout Design at Salh Intersection - A case study

Dr. Maninder Singh1 Rajbir Kaur

2

1. Assistant Professor, Department of Civil Engineering, Punjabi University Patiala

2. M. Tech. Scholar, Transportation Engineering, Punjabi University Patiala

Abstract: Roundabout is a specialized form of at grade intersection which allows the movement of traffic

in one direction around the central island and move out of the roundabout into their desired direction. In

this research work, traffic volume count study, spot speed study, level of service are determined and

design of roundabout at Salh intersection Ludhiana, Punjab as per IRC:65-2017 is done. Traffic volume

count study found out that traffic volume at the site is 3362 PCU/hr, thus roundabout is required at the

site. From spot speed study, mean speed obtained at BRS bypass road, Phullanwala chowk road, City

centre road and Ramgarhia road are 31.6, 28.4, 30.96 and 34.11 kmph respectively. Thus, design speed

for the roundabout is taken as 30 Kmph. Level of service at the site is calculated by vehicular delay model

as per IRC: 65-2017. Vehicular delay is 29.96 seconds, thus level of service at the site is D. Level of

service D indicates unstable flow, with tolerable operating speeds being maintained though considerably

affected by changes in operating conditions. Fluctuations in volume and temporary restrictions to flow

may cause substantial drop in operation speeds.

Keywords: Traffic volume count study, Spot speed study, Level of service, Roundabout

1. Introduction

Intersection is defined as the area where two or more roads meet or cross. If two or more roads meet or

cross at same level, it is called at grade intersection. If two or more roads meet or cross at different level

is called grade separated intersection [1]. Roundabout is a specialized form of at grade intersection which

allows the movement of traffic in one direction round a central island in an organised manner and move

out of the roundabout into their desired direction. When the vehicle from the approaching road reaches

the roundabout, it waits to accept the gap, priority is given to circulating traffic, so the roundabout

operates on the gap acceptance and priority rule.

Roundabout is an enlarged junction, where all entering vehicles shall give way and find suitable gaps to

move around the central island in one direction before they move out of the traffic flow into their

respective directions radiating from the island [2].

The main objective of this study is to conduct traffic volume count study, spot speed study,

level of service and design of roundabout at Salh intersection Ludhiana, Punjab. These studies are

conducted to know the existing traffic conditions at the site. Traffic volume data is collected by manual

method. Spot speed study is conducted by finding the time taken to cover a distance of 10 m by using stop

Journal of University of Shanghai for Science and Technology ISSN: 1007-6735

Volume 22, Issue 11, November - 2020 Page-931

watch, then speed is calculated. Level of service is calculated by using vehicular delay model as per IRC:

65-2017. Roundabout is designed by following IRC: 65-2017 guidelines.

2. Literature Review

Roundabout emerged from traffic spheres that were constructed in the past. In 1909, first British

roundabout was built in Letchworth Garden city [3]. Rapid automobile technology developed to allow

higher speeds through the intersections. With increase in traffic volume, more and more traffic circles

locked up so they were replaced with signalised intersections. Priority to the circle rule evolved, which

increased the capacity of roundabout and reduced delays [4]. A new type of roundabout known as turbo-

roundabout came into being, which is a modified form of the traditional roundabout having multi lanes in

which motorist obey a particular roadway in order to reach their planned place. The motorway comprises

of consistent twisted track, kerbs are provided to divide the lanes at the entrance, round and departure

area. Turbo roundabout decreases the number of conflict points and provides safety [5]. Another study

found that motorcade functioning and capacity is increased on roundabouts with improper flow of traffic

when provided with metering arrangement. Metering signals can improve the performance of roundabout.

Metering can reduce delay time as compared to non-metering [6]. Roundabouts work without power,

lights are required only in bad weather or in the night [7]. The highest number of automobile that can be

accommodated at a given point on a roundabout during a particular time at a roundabout with signals is

calculated mainly by the interval acceptance concept. Roundabout functions on gap acceptance. Gap

acceptance is when motorist remains at their position till they find a suitable space to enter the roundabout

when signals are not provided at the roundabout. Gap acceptance concept is used to study the maximum

number of automobile that can pass a given point during a given time on roundabout, which involves

changing features and prime concern regulation [8]. Nowadays, many single lane roundabouts have a

problem of capacity, which can be solved by means of changing them with multi lane roundabouts [9].

Intersections control various vehicles which can move simultaneously on the associated streets [10]. The

conduct of overwhelming vehicles is not quite the same as the conventional traveller vehicles because of

their enormous size and lower increasing speed/deceleration execution [11]. Traffic circles can be

characterized by number of circulatory and passage paths [12]. The nearness of passerby crosswalks and

cyclists at inventive arrangements of traffic circles may bring about an impedance impact on the

accessible vehicular limit of the convergence [13]. Over 65% of the mishaps that happened on the traffic

circles in various places of the world happened during arrangements at the passage of the traffic circle,

loosing control at the section and backside crashes when vehicles are halted out of nowhere at the exit

[14].



3. Site selection

Salh intersection Ludhiana, Punjab is selected for design of roundabout because traffic is unorganized due

to which accidents occur here and traffic is expected to increase in future. Roundabout will organize the

traffic flow at the site. Ludhiana is a metropolitan city and most midway found area in Punjab. Ludhiana

is most populated city in Punjab [15]. Vehicles are required to reduce their speed in order to negotiate the

roundabout, thus accidents will be reduced at the site. Roundabout will help in reducing the traffic

problems at the site by regulating the traffic flow. Roundabout may be a good choice of traffic

management in an at-grade intersection. Roundabout helps in reducing delay and providing better safety.

This site is suitable for roundabout because it has high proportion of right turning traffic. Roundabouts

can work efficiently with high volumes of right turning traffic.

4. Traffic studies and results

4.1. Traffic volume count study

Traffic volume is the number of automobile moving in a specified direction on a given roadway that pass

a given point during specified unit of time. Traffic volume is collected by manual count method during

peak hours at the site. Image of Salh intersection is shown in figure 1. Traffic volume study by manual

count includes employing of field team of enumerators at the site. Traffic volume data is collected by

enumerators at the 4 arms of intersection by classifying the type of vehicles on the sheet at specified time

intervals during specified days of week. Traffic volume is then converted to PCU in mixed traffic flow.

Traffic flow diagram of Salh intersection is shown in figure 2. Traffic volume count is done for 6 hours a

day, 2 hours each in morning, afternoon and evening for 3 weeks on Sunday, Monday, Friday as Sunday

indicates the traffic during weekend, Monday and Friday shows traffic during first and last working day

and have maximum traffic. An overall idea of the traffic conditions at the site can be formed. Traffic

Volume data is collected from 2 February, 2020 to 21 February, 2020. PCU values used in the traffic

volume count at Salh intersection are given in Table 1. Traffic volume count study is conducted at BRS

bypass, Phullanwala chowk, City centre road and Ramgarhia road is shown in table 2, 3, 4 and 5

respectively.



Table 1. PCU values used in traffic volume count study

S. No. Vehicle class PCU Value

1. Passenger Car 1.0

2. Bus, truck 3.7

3. Motorized two wheeler 0.32

4. Motorized three wheeler 0.83

5. Cycle 0.18

6. Light commercial vehicle 1.88

7. Cycle rickshaw 1.12

8. Hand cart 2

9. Horse cart 3

10. Tractor-trailer 4

Figure 1. Image of Salh intersection Ludhiana, Punjab



Table 2. Traffic volume at BRS bypass road

BRS Bypass

side

Time Total vehicles in PCU

First Second Third

S M F S M F S M F

BRS Bypass

to Phullanwala

chowk

M (8-10)AM 71 106 120 75 111 133 83 109 144

A (12-2) PM 93 180 196 101 180 216 99 176 211

E (4-6)PM 97 94 122 103 109 138 111 112 142

BRS Bypass

to City Centre

Road

M (8-10)AM 487 772 584 514 762 733 525 807 824

A (12-2)PM 603 822 738 621 838 808 647 817 811

E (4-6)PM 691 613 805 657 649 987 714 671 972

BRS Bypass

to Ramgarhia

Road

M (8-10)AM 144 96 177 153 105 214 161 123 254

A (12-2)PM 167 179 219 175 185 228 183 172 239

E (4-6)PM 103 133 190 114 128 202 123 136 207

Total

M (8-10)AM 702 974 881 742 978 1080 769 1039 1222

A (12-2)PM 863 1181 1153 897 1203 1252 929 1165 1261

E (4-6) PM 891 840 1117 874 886 1327 948 919 1321



Table 3. Traffic volume at Phullanwala chowk road

Phullanwalla

chowk side

Time

Total vehicles in PCU

First Second Third

S M F S M F S M F

Phullanwala chowk

to BRS Bypass

M (8-10)AM 179 118 125 183 124 145 193 122 155

A (12-2) PM 97 206 215 106 205 236 105 201 231

E (4-6)PM 86 101 135 93 114 154 101 122 159

Phullanwala chowk

to City Centre Road

M (8-10)AM 519 848 678 548 840 833 558 883 924

A (12-2)PM 580 824 801 593 850 874 614 821 879

E (4-6)PM 610 643 729 638 688 911 690 710 895

Phullanwala

chowk to

Ramgarhia Road

M (8-10)AM 132 103 200 141 116 237 149 128 272

A (12-2)PM 150 183 249 153 192 259 166 176 267

E (4-6)PM 116 132 213 129 136 223 142 142 231

Total

M (8-10)AM 830 1069 1003 872 1080 1215 900 1133 1351

A (12-2)PM 827 1213 1265 852 1247 1369 885 1198 1377

E (4-6)PM 812 876 1077 860 938 1288 933 974 1285



Table 4. Traffic volume at City centre road

City Centre

side


First Second Third

S M F S M F S M F

City centre road

to BRS Bypass

M (8-10)AM 133 220 159 155 227 176 164 233 180

A (12-2) PM 122 101 181 124 123 196 137 132 200

E (4-6)PM 157 144 202 176 158 253 192 155 266

City centre

road to

Phullanwala

chowk

M (8-10)AM 253 333 366 271 371 401 263 388 409

A (12-2)PM 149 301 229 170 323 267 181 333 306

E (4-6)PM 187 361 311 190 380 324 203 382 371

City centre road

to Ramgarhia

Road

M (8-10)AM 111 245 229 114 242 237 107 255 237

A (12-2)PM 81 202 205 81 228 231 88 233 271

E (4-6)PM 103 207 162 106 252 177 122 253 187

Total

M (8-10)AM 497 799 755 540 840 814 535 877 826

A (12-2)PM 352 604 615 375 674 693 406 699 777

E (4-6)PM 447 712 675 472 790 755 517 790 823



Table 5. Traffic volume at Ramgarhia road

Ramgarhia road

side


First Second Third

S M F S M F S M F

Ramgarhia road

to BRS Bypass

M (8-10)AM 122 222 227 124 216 235 118 227 233

A (12-2) PM 102 129 171 103 154 201 109 161 240

E (4-6)PM 93 192 177 96 236 194 123 237 203

Ramgarhia road

to Phullanwala

chowk

M (8-10)AM 272 374 350 291 413 383 283 431 392

A (12-2)PM 183 263 193 205 283 231 215 298 267

E (4-6)PM 245 371 349 253 391 362 275 393 405

Ramgarhia road

to city centre

road

M (8-10)AM 146 235 170 162 233 187 171 236 187

A (12-2)PM 105 106 194 110 128 209 123 138 213

E (4-6)PM 166 136 177 183 148 227 198 145 239

Total

M (8-10)AM 540 830 747 576 862 804 571 894 812

A (12-2)PM 389 498 558 418 565 640 446 597 719

E (4-6)PM 504 699 704 531 774 783 596 775 847

S: Sunday M: Monday F: Friday M: Morning A: Afternoon E: Evening



Figure 2. Traffic flow diagram at Salh chowk Ludhiana, Punjab

4.2. Spot speed study

Spot speed is the immediate speed of a vehicle at a point on road. There are many methods of conducting

spot speed study. Equipments used for spot speed measurements are graphic recorder, electronic meter,

photo electric meter, photographic methods and using radar speed meter. In this research work, spot speed

of a vehicle is measured by finding time taken to cover a distance of 10 m. The selected distance (d, m) is

marked on pavement surface and the time taken (t, sec) for a vehicle to cover this distance is measured by

using a stop watch. One observer stand at starting point of marked surface and another observer at end

point of marked surface. When a vehicle passes the starting point of marked surface, observer starts the

City centre

road

Ramgarhia road

Phullanwala

chowk road

BRS

bypass

road

1069

279

143

262

63

8

72

1

17

2

73

7

18

7

17

6

1

75

60

0

183

161

166

1055

Total incoming traffic

= 3362 PCU/hr

Total outgoing traffic

= 3362 PCU/hr

Prem vihar

park

Car studio

sodhi car

bazaar

Hotels Shopping

centre



stop watch and when it passes the end point of marked surface, observer at end point gives indication to

observer at starting point to stop the stop watch. In this way, time taken by a vehicle to cover the marked

distance is calculated.

Speed of a vehicle is calculated by a given formula:

Speed v = d / t (m / sec)

= d / t x 18/5 (Kmph)

Spot speed study is done for three hours in the afternoon at different arms of intersection each day

because most of the work places close at this time and people go back to their home. Spot speed study is

conducted at BRS bypass, Phullanwala chowk, city centre road and Ramgarhia road is shown in table 6,

7, 8 and 9 respectively.

Table 6. Spot speed analysis of BRS bypass road

Time: 3:30 to 6:30 pm Date: 2/03/2020

Speed Groups (Kmph) Frequency Frequency Cumulative

Frequency

Cumulative

Frequency

Computation

Lower

limit

Mid limit Upper

limit

Xi Fi % ∑Fi % FiXi

18 21 24 35 30 35 30 735

24 27 30 21 18 56 48 567

30 33 36 22 19 78 67 726

36 39 42 17 15 95 82 663

42 45 48 10 8 105 90 450

48 51 54 6 5 111 95 306

54 57 60 1 0.8 112 95.8 57

60 63 66 1 0.8 113 96.6 66

N=∑fi=113 ∑FiXi=3570

Mean speed = ∑ FiXi / N



= 3570 / 113 = 31.6 Kmph

Table 7. Spot speed analysis of Phullanwala chowk road

Time: 3:30 to 6:30 pm Date: 3/03/2020

Speed Groups (Kmph) Frequency Frequency Cumulative

Frequency

Cumulative

Frequency

Computation

Lower

limit

Mid limit Upper

limit


16 18 20 11 12 11 12 198

20 22 24 18 19 29 31 396

24 26 28 18 19 47 50 468

28 30 32 13 14 60 64 390

32 34 36 21 23 81 87 714

36 38 40 3 3 84 90 114

40 42 44 3 3 87 93 126

44 46 48 4 4 91 97 184

N=∑fi=91 ∑FiXi=2590


= 2590 / 91 = 28.4 Kmph



Table 8. Spot speed analysis of city centre road

Time: 3:30 to 6:30 pm Date: 4/03/2020

Speed Groups

(Kmph)

Frequency Frequency Cumulative

Frequency

Cumulative

Frequency

Computation

Lower

limit

Mid

limit

Upper

limit


19 21 23 17 17 17 17 357

23 25 27 19 19 36 36 475

27 29 31 18 18 54 54 522

31 33 35 10 10 64 64 330

35 37 39 25 25 89 89 925

39 41 43 5 5 94 94 205

43 45 47 3 3 97 97 135

47 49 51 3 3 100 100 147

N=∑fi=100 ∑FiXi=3096


= 3096/ 100 =30.96 Kmph



Table 9. Spot speed analysis of Ramgahia road

Time: 3:30 to 6:30 pm Date: 5/03/2020

Speed Groups

(Kmph)

Frequency Frequency Cumulative

Frequency

Cumulative

Frequency

Computation

Lower

limit

Mid

limit

Upper

limit


17 20 23 20 19 20 19 400

23 26 29 18 17 38 36 468

29 32 35 19 18 57 54 608

35 38 41 15 14 72 68 570

41 44 47 20 19 92 87 880

47 50 53 4 3 96 90 200

53 56 59 3 2 99 92 168

59 62 65 3 2 102 94 186

N=∑fi=102 ∑FiXi=3480


= 3480 / 102 = 34.11 Kmph

4.3. Level of service

Level of service is the opportunity that the road can provide under different operating characteristics and

traffic volumes.

Level of service is calculated as per IRC: 65-2017.

The calculation of level of service is based on vehicular delay model. It takes into consideration the delay

experienced by the vehicle from entry point to reach the exit point of the roundabout. The relationship



between approach traffic flow and average vehicular delay is shown in figure 3. Level of service for

different delay is shown in table 10.

The delay model empirically derived is given as:

y = 0.8*e0.001x

Total approach traffic flow at the intersection in vehicle/hour is 3623. It is obtained from traffic volume

count study.

= 0.8*e0.001x3623

= 0.8*e3.623 = 0.8 x 37.45 = 29.96 seconds

where,

y = vehicular delay in seconds

x = total approach traffic flow in vehicle/hour

Figure 3. Relationship between approach traffic flow and average vehicular delay

Table 10. Level of service for different delay

Level of service Average delay d per veh in sec

A

Vehicular delay is 29.96 seconds. From the above table, level of service at the intersection is D.

4.4. Design of roundabout

Design of roundabout is shown in table 11. Dimensions of roundabout components are described in figure

4. Design of roundabout as per IRC: 65-2017 is given by:

1) Design speed- It is calculated by spot speed study. Mean speed calculated at BRS bypass road,

Phullanwala chowk road, City centre road and Ramgarhia road are 31.6, 28.4, 30.96 and 34.11 kmph

respectively. Thus, design speed for the roundabout is taken as 30 Kmph.

2) Inscribed circle diameter- For urban double lane roundabout, inscribed circle diameter should be

between 40 to 70 m as per IRC:65-2017. Thus, it is taken as 40 m.

3) Central island diameter- Central island diameter should be 24 m for 40 m inscribed circle diameter as

per IRC:65-2017.

4) Width of circulatory carriageway- Width of circulatory carriageway should be 8 m for 40 m inscribed

circle diameter and 24 m central island diameter.

5) Entry width- It must be 6.5 m for radius at entry and exit curve 20 to 40 m. Entry width of 6.5 m is

adopted.

6) Exit width- It must be 6.5 m for radius at entry and exit curve 20 to 40 m. Exit width of 6.5 m is

adopted.

7) Radius at entry curve- It should be between 20 to 40 m for 2 lane road. Radius at entry curve is taken

as 20 m.

8) Radius at exit curve- It must be between 20 to 40 m for 2 lane road and greater than entry radius as

vehicles should be able to accelerate on the exit. Radius at exit curve is taken as 30 m.

9) Entry angle- It should lie between 20 and 60 and should be greater than exit angle. Entry angle is taken

as 30.

10) Exit angle- It should be smaller than entry angle. Exit angle is taken as 20.

11) Weaving length- Its recommended value is 45 to 90 m for design speed of 40 kmph and 30 to 60 m

for 30 kmph speed. The design speed is 31 kmph, thus 30 m weaving length is adopted.

12) Camber- Camber of order 0.5% to 1% can be provided to drain the surface runoff away from central

island. Camber of 0.5 % is adopted.

13) Sight distance- Recommended value of sight distance for design speed 30 kmph is 30 m.

14) Kerb- It shall not be more than 0.225 m. The value of kerb is taken as 0.225 m.

15) Splitter island- It should be at least 2.4 m wide.



Table 11. Design of roundabout

S. No. Elements Dimensions

1. Design Speed 30 Km/h

2. Central island diameter 24 m

3. Inscribed circle diameter 40 m

4. Width of circulatory carriageway 8 m

5. Entry width 6.5 m

6. Exit width 6.5 m

7. Radius at entry curve 20 m

8. Radius at exit curve 30 m

9. Entry angle 30

10. Exit angle 20

11. Weaving length 30 m

12 Camber 0.5%

13. Sight distance 30 m

14. Kerb 0.225 m

15. Splitter island 2.4 m

=20m

=30m

=6.5m

=6.5m

= 8m

=40m

Central island dia

= 24m

Weaving

width Weaving length = 30 m



Figure 4. Diagram showing dimensions of roundabout components

5. Conclusions

This study concludes that traffic volume count study at Salh intersection Ludhiana, Punjab shows that

total incoming traffic at the intersection is 3362 PCU/hr and total outgoing traffic at the intersection is

3362 PCU/hr, thus roundabout is required at the site so as to control and guide the traffic flow. From spot

speed study, mean speed calculated at BRS bypass road, Phullanwala chowk road, City centre road and

Ramgarhia road are 31.6, 28.4, 30.96 and 34.11 kmph respectively. Thus, design speed for the roundabout

is taken as 30 Kmph. Design speed is the main element on which geometric design elements depends.

Important geometrical elements such as sight distance, weaving length etc. are determined on the basis of

design speed. Level of service at the site is D. Level of service D indicates unstable flow, with tolerable

operating speeds being maintained though considerably affected by changes in operating conditions.

Fluctuations in volume and temporary restrictions to flow may cause substantial drop in operation speeds.

Drivers have little freedom to manoeuvre: comfort and convenience are low, but conditions can be

tolerated for short periods of time. Roundabout has been designed by following IRC: 65-2017 guidelines.

A well designed roundabout can prevent the traffic problems in future.

References

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