An Approach for the Estimation of Entry Flows on Roundabouts Transportation Research Procedia 17 ( 2016 ) 52 – 62 2352 1465 © 2016 The Authors Published by Elsevier B V This is an open access article[.]
Available online at www.sciencedirect.com ScienceDirect Transportation Research Procedia 17 (2016) 52 – 62 11th Transportation Planning and Implementation Methodologies for Developing Countries TPMDC 2014, 10-12 December 2014, Mumbai, India An approach for the estimation of entry flows on roundabouts Srinath Mahesha , AbdullahAhmada, Rajat Rastogia,* a Department of Civil Engineering, Indian Institute of Technology Roorkee, ROORKEE - 247667 INDIA Abstract Roundabouts are commonly used as a means of intersection control for moderate traffic flows and junctions having variations in the intersection geometry It facilitates an orderly movement of traffic in a circular motion around a central island which is generally circular in shape The circulating traffic is considered to be the priority stream and entering traffic waits for a suitable gap in the circulating traffic In this fashion, it reduces the stopped delays as observed on the signalized intersections, as well as reduces the crashes and expenditure required for maintenance of traffic signals This paper examines the entry capacity of a roundabout under different circulating flows by measuring the field entry flows Queue formation in the approach is taken as an indicator that the approach is operating at capacity Relationship between entry flow and circulating flow is found following negative exponential distribution This is compared with the entry capacity estimations based on HCM 2010 The field entry flows are found to be higher than that given by the HCM 2010 equation This is due to the lower critical gap acceptance behavior shown by the Indian drivers under mixed traffic compared to the homogenous traffic modeled in the HCM 2010 An adjustment factor, multiplicative to the entry flow estimation based on HCM 2010 equation, is proposed which can be used by field engineers directly This will be simpler than the approach based on critical gap computation, which is tedious and difficult © 2016 The Authors Published by Elsevierby B.V This is anB.open © 2015 The Authors Published Elsevier V access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the Department of Civil Engineering, Indian Institute of Peer-review under responsibility of the Department of Civil Engineering, Indian Institute of Technology Bombay Technology Bombay Keywords: Roundabouts, entry capacity, critical gap, HCM 2010 ; * Corresponding author Tel.: +91-13-3228-5447 E-mail address: rajatfce@iitr.ac.in 2352-1465 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the Department of Civil Engineering, Indian Institute of Technology Bombay doi:10.1016/j.trpro.2016.11.060 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 Introduction Roundabouts are used as intersection control measures for a number of traffic and site conditions as they not require any active control in terms of the presence of a traffic police They are ideal at locations having more than four approach legs intersecting at acute angles and with sufficient space for the central island For moderate traffic, roundabouts increase the traffic handling capacity of an intersection and improve its performance by reducing delays and crashes Reduction in the number and severity of crashes is due to the decrease in the number of conflict points when an uncontrolled intersection is converted into a roundabout The traffic enters a roundabout after seeking a suitable gap in the circulating stream of vehicles thereby, the crossing conflicts which are the most severe are completely eliminated and converted to merging and diverging Quantitatively, the number of conflict point reduces from 32 in a Two-Way Stop Controlled (TWSC) intersection to in the case of a roundabout Besides, during low flows, there is less likelihood of crashes due to over speeding vehicles as there are inherent geometric features in the approaches to a roundabout which discourage high vehicle speeds in approaches The traffic handling ability of a roundabout largely depends on its geometry and to lesser extent on driver behavior The geometric elements include the diameter of the central island, width at entry, width at exit, width of the circulating roadway, weaving width, weaving length, etc The driver behavior affects the performance of a roundabout and it is taken into account by introducing the independent variables, namely critical gap and follow-up time The critical gap is defined as that gap in-between the circulating vehicles which would be considered appropriate by most of the drivers to enter into the circulating traffic It would depend upon the composition and volume of the entering traffic, as well as, the circulating flow The presence of pedestrians and cyclists on a roundabout leads to a decrease in its traffic handling capacity A provision of separate path for these road users would improve both the safety of these vehicles or users and traffic handling efficiency of the roundabout Literature Review Earlier studies on development of models for estimation of entry flow or approach capacity at a roundabout can be broadly classified into two categories: one empirical models and other analytical models Empirical models like the Indian, U.K and Swiss methods are based on direct field data collected at the approaches to roundabouts, whereas, the analytical models like the U.S., German and Australian models are based on gap acceptance behavior of the drivers The Indian formula (IRC-65 1976) for estimation of capacity of a rotary is based on Wardrop’s equation which is empirical in nature and takes into account the geometric elements like entry or exit width, length and width of weaving section, and the proportion of weaving traffic with respect to the total traffic in weaving section The entire roundabout is divided into separate weaving sections and the practical capacity of the roundabout is synonymous with the weaving section having the least capacity The U.K method is based on the formula proposed by Transport and Road Research Laboratory (TRRL) It takes into account the geometric parameters like entry width, flare length, sharpness of the flare, entry bend radius, entry angle, inscribed circle diameter, etc (Kimber 1980) The Swiss method is similar to the U.K method but considers the effect of exiting traffic in the direction opposite to the entering traffic (Bovy et al 1991) In contrast, the HCM (2010) proposes an analytical approach based on critical gap and follow-up time to determine the entry capacity of a roundabout The recommended values of critical gap and follow-up time are 4.1 – 4.6 s and 2.6 – 3.1 s respectively The general equation is modified for roundabouts with different combinations of entry lanes and circulating lanes It is also suggested to verify the critical gap and follow-up time at important locations in conditions different from those found in U.S.A The Australian formula given by Akcelik et al (1998) gives capacity lane by lane The entry lane having the largest entry flow is referred to as the dominant entry lane and the lane having the smallest entry flow as the subdominant entry lane Brilon et al (1997) proposed a formula which takes into account the circulating flow in front of the entry, number of circulating lanes, number of lanes in the subject entry, minimum headway between the circulating vehicles, critical gap and follow-up time The critical gap, follow-up time and the headway between the circulating vehicles are collectively referred to as the psycho-technical times and have been estimated as 4.12 s, 2.88 s and 2.1 s respectively, for German conditions Al-Masaeid and Faddah (1997) developed an empirical model for estimating roundabout entry capacity for conditions in Jordon The research concluded that the entry width and the central island diameter have the greatest 53 54 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 effect and the circulating roadway width has the least effect on estimated entry capacity as compared to other geometric variables Polus and Shmueli (1997) studied six small to medium-sized roundabouts with single circulating lane in urban and suburban areas in Israel The developed model showed an exponential decrease in entry capacity with an increase in the circulating flow Polus et al (2003) studied the effect of waiting time of the approaching drivers on the critical gap acceptance behavior of the drivers and found that as the circulating flow changes, the critical gap also changes At high circulating flows, the effect of critical gap on capacity was found to be more significant and at low circulating flows, the central island diameter was the factor which affected the entry capacity The methods discussed in the preceding paragraphs vary in one dimension i.e complexity There are some methods which are not based on any theoretical foundation like the method in use in India for capacity estimation of rotaries Other methods considers large number of variables so as to incorporate influences of geometry, traffic flow and driver behavior This makes the estimation cumbersome HCM method is simpler as far as the number of variables is concerned But the estimation of these variables is tedious There are many methods to estimate critical gaps and the applicability of one out of all always remains questionable Moreover, implementing agencies may found it difficult to estimate critical gaps and follow-up-times at different locations and in different cities because these are traffic and location specific In such a condition a simpler approach is needed which is usable by people working in different walks of life This paper attempts to provide one such approach Data Collection Data collection was done in the city of Chandigarh, capital city of both, the states of Punjab and Haryana Most of the intersections in the city are in the form of roundabouts having four approaches which are mutually perpendicular following the grid pattern Following criteria are considered for the selection of a roundabout: x x x x x Four- approaches which are mutually perpendicular Uncontrolled traffic operation, i.e not having a traffic signal or police personal Flat longitudinal gradient on all of the approaches Negligible interference by pedestrians and cyclists Availability of a multi-storey building nearby for placing the camera Two roundabouts were selected for the study purpose which fulfilled the above-mentioned criteria The traffic and operational conditions of the two roundabouts are shown in Figure The locational details and inventory data of the selected roundabouts are given in Table The roundabouts are named as R25 and R37, where ‘R’ stands for a roundabout and number (25 or 37) represents the diameter of the central island in meters Figure Traffic and operational condition of roundabouts 55 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 Table Inventory data of selected roundabouts Name of the roundabout Geometric element R 25 (Sector 22-23-35-36) R 37 (35-36-42-43) 25 37.5 Inscribed Circle Diameter (m) 41 51.5 Entry Width (m) 8.5 Exit Width (m) 8.5 Approach Width (m) 6.7 7.5 Circulating Roadway Width (m) Central Island Diameter (m) The data collected can be broadly classified into two categories: inventory data and traffic volume data The inventory data includes the geometric details of the roundabout like central island diameter, entry width, approach width, weaving width, weaving length, etc The inventory data was collected using a tape or a measuring wheel Traffic volume data are taken by vehicle category both at the entry of an approach and on the circulating section, thus giving information regarding total traffic volume and its composition on the two sections This data were collected using a video camera which was installed at the top of a nearby building The data collection was done in the months of September and November 2013, which are considered to be the normal months as the traffic flow is least affected by environmental influences during this period The video was captured from a.m to 12 a.m and p.m to p.m on a typical clear weekday The PCU values for converting the heterogeneous traffic into a homogeneous one are taken from IRC 65: 1976 For conversion into pcu, two wheelers are 0.75 pcu, autos are 1.0 pcu and buses are assessed as 2.8 pcu The composition of entering traffic is shown in the form of a pie-chart in Figure The traffic volume data from different approaches and in different directions (right, left or through) are given in Table 1% 3% Car 16% TW 36% Auto Bus 44% Others Figure Composition of entering traffic from an approach Table Traffic volume data at the selected roundabouts Name roundabout R 25 R 37 of Direction Entry volume (pcu/h) RT Total (pcu/h) volume Circulating volume (pcu/h) LT ST N 570 1458 847 2875 1445 E 289 748 698 1735 2847 S 487 749 425 1661 2293 W 245 478 542 1265 1872 N 475 412 578 1465 1620 E 248 398 247 893 1468 S 214 471 488 1173 1223 W 147 654 478 1273 1206 56 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 Analysis and Results The traffic flow values are estimated in both, vehicles per hour and pcu/h These estimated values of traffic flow at an entry from an approach and in circulating area in front of it are shown in Figure and for the two roundabouts respectively As mentioned before, the entry flows and circulating flows corresponds to the time period during which there is queue formation in the approach Queue formation represents the saturated condition of that approach The duration and composition of the queue is also recorded along with the corresponding circulating flow 2500 y = 2729e-2E-04x R² = 0.7154 Entry flow 2000 1500 Vehicles/Hour y = 2986.5e-3E-04x R² = 0.7706 1000 PCU's/Hour 500 1000 1500 2000 2500 3000 Circulating flow 3500 4000 Figure Entry flow versus circulating flow for R 25 roundabout It can be noted that, irrespective of the measurement form for flows, the entry flow reduces exponentially with an increase in the circulating flow The functional forms are found to be satisfactory or above with respect to R-square value of the estimated equations This is found similar to the one reported by Polus and Shmueli (1997) The relationship is supporting the normal belief that when the flow on circulating lanes is low, more number of vehicles can move in from an approach, but as vehicles add to the circulating flow lesser number of vehicles can move in Relative position of the plots in terms of vehicles per hour and pcu/h indicate towards the absence of big size vehicles in the flow Figure presents a comparison between the entry capacities (in vehicle per hour) of the two roundabouts The entry capacity of a larger central island diameter roundabout is found to be higher than that of a relatively smaller central island diameter roundabout The width of the entry approach and circulating section are more or less similar (can be defined as two lane system), and the traffic composition is also equivalent Still there is an increase in the entry capacity of the second roundabout This can only be attributed to the influence of increase in diameter of the central island, which causes an increase in the space within the weaving section, thus accommodating more traffic volume This is supported by the work of Al-Masaeid and Faddah (1997) done in Jordon 57 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 3000 y = 3487.3e-2E-04x R² = 0.742 2500 vehicles/Hour Entry flow 2000 PCU's/Hour 1500 y = 3633.2e-3E-04x R² = 0.8554 1000 500 1000 1500 2000 2500 Circulating flow 3000 3500 Figure Entry flow versus circulating flow for R 37 roundabout 3000 Entry flow (veh/h) 2500 2000 D = 37.5 m 1500 D = 25 m 1000 500 0 500 1000 1500 2000 2500 Circulating flow (veh/h) 3000 3500 4000 Figure Comparison between entry capacity and circulating flow Next, the estimated values of entry flows with respect to the circulating flows for the two roundabouts are compared with the estimates obtained on using HCM 2010 equation The HCM (2010) roundabout entry capacity model is expressed as given by equation (1) Ce = A * e B*Vc (1) Where, A= B= 3600 tf t c 0.5 * t f 3600 Vc = conflicting flow rate in pcu/h (2) (3) 58 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 tf = follow-up time (s) tc = critical gap (s) As traffic volume in HCM equation is considered in pcu/h, the traffic on the two roundabouts is re-estimated using the guidelines of IRC:65 as already mentioned The relationships and comparison are shown in Figure It can be noted that the proposed entry capacity of an approach based on HCM 2010 equation is quite low as compared to the Indian field conditions This can be attributed to two factors: one the traffic heterogeneity in Indian condition as compared to homogenous condition in the US, and the difference in driver behavior in two countries The mix of the traffic and the absence of large size vehicles in present condition allow sharing of road not on the basis of lane discipline but based on size of the vehicles The share of 44% of two-wheelers not require a 3.5 m wide lane, rather two streams of two-wheelers can get accommodated in this much wide section of the road Therefore, the number of vehicles in the approaches and circulating area will be more than those when lane-based movement is followed Another reason is the difference in behavior of the Indian drivers as compared to their American counterparts in terms of gap acceptance Besides, the higher proportion of two wheelers in the Indian traffic, the drivers of different category of vehicles are observed accepting lower gap sizes as compared to those in the US This also led to an increase in the entry capacity In order to understand the difference in the merging behavior of Indian drivers as compared to the US regarding traffic flows at roundabouts, the critical gap and follow-up times are computed at both the locations The critical gap and follow-up time are the independent variables on which the entry capacity is dependent The average value of critical gap and follow-up-time is taken as 4.5 s and 2.7 s (60% of critical gap) in the HCM 2010 The estimation of critical gap is done using Modified Raff’s method, which utilizes both the accepted and rejected gaps The followup-time is estimated from the video of the two sites The results are given in Table It can be noted that for none of the vehicle types, the value of critical gap or follow-up-time are anywhere near to the values being adopted by HCM 2010 Even in the case of car, these are around 50 to 55% of those used by HCM In the case of two-wheelers, these are around 40% only These values are also different than those observed in Germany, another developed country with homogenous traffic (Brilon et al 1997) These clearly indicate that the drivers in Indian condition are relatively impatient in nature This may be the influence of more number of vehicles sharing the same road space and trying to move without stopping Another factor is the squeezing effect of small size vehicles in-between larger size vehicles and on lanes The ratio between follow-up time and critical gap (rt) is also computed for different category of vehicles on two roundabouts It is found to be less than 0.60, which is different than the ranges being reported in literature by different researchers (Brilon 1988; Hagring et al 2003; Tian et al 2000) They have reported this ratio as 0.60 and above Table Critical gap and follow-up times for vehicles at the two roundabouts Roundabout R 25 R 37 Vehicle type Critical Gap (s) Follow-up time (s) rt Two-wheeler Car 1.85 1.09 0.59 2.43 1.43 Three-wheeler 0.59 2.12 1.29 0.61 0.57 Two-wheeler 1.82 1.03 Car 2.51 1.43 0.57 Three-wheeler 2.17 1.26 0.58 Once the values of critical gap and follow-up time are estimated for Indian traffic flow conditions, then these are used to find the traffic stream critical gap for the approach The stream critical gap and follow-up time are computed using weighted average approach The percent value of the composition of traffic is used as weight for the respective value of critical gaps and follow-up-times so as to arrive at the traffic stream value This gave a value of 2.2 s for critical gap and 1.2 s for follow-up time Using these values in the equation proposed by HCM 2010, the entry capacity was re-estimated with respect to the same values of circulating flows as used in previous analysis This variation between the two variables is then compared with the field data available for the two roundabouts These are also shown in Figure It can now be noted that the difference between the field values under heterogeneous and 59 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 adjusted-homogenous traffic has reduced on account of use of real values But still the HCM 2010 equation is not able to replicate the traffic conditions existing under Indian conditions Therefore, there is a need to use a multiplicative adjustment factor which can raise the estimation using HCM 2010 equation to satisfy the Indian traffic condition 3000 Entry flow (pcu/h) 2500 D = 37.5 m 2000 D = 25 m 1500 Using computed tc and tf values 1000 500 HCM 2010 0 1000 2000 3000 Circulating flow (pcu/h) 4000 Figure Comparison of field entry capacity with HCM 2010 and adjusted HCM 2010 for Indian Condition The adjustment factor is proposed for the entry flow at different circulating flow values for both sizes of roundabouts as they represent different flow behaviours Looking at the cumbersome process of estimation of critical gaps and follow-up-times, two ranges of adjustment factors are suggested in this paper First set of adjustment factors are estimated with respect to the original HCM equation which considers critical gap as 4.5 s and follow-up-time as 2.7 s Second set of adjustment factors correspond to the adjusted HCM equation i.e one which considers actual values of critical gap (2.2 s) and follow-up-time (1.2 s) for traffic stream in India The adjustment factor is defined as the ratio between the field entry flow value and that given by HCM equation or adjusted HCM equation The adjustment factors are shown in Figure and Figure 8, respectively Adjustment factor Central Island D = 37.5 m D = 25 m 0 1000 2000 Circulating flow (pcu/h) 3000 Figure Adjustment factor with respect to HCM 2010 4000 60 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 2.5 y = 1.25367e0.00016x R² = 0.99953 Adjustment factor 1.5 Central Island 1.02789e0.00016x y= R² = 0.99960 D = 37.5 m 0.5 D = 25 m 0 1000 2000 3000 Circulating flow (pcu/h) 4000 Figure Adjustment factor with respect to adjusted HCM 2010 equation It is clear from the two figures that the variation in adjustment factor with an increase in circulating flow becomes quite large, from 2.0 to 5.5 for R25 roundabout and from 2.5 to 6.8 for R37 roundabout, when the original HCM equation for estimation of entry capacity is considered This is because of wide variation in the traffic flows in two countries As the flow conditions prevailing in India are embedded in the HCM equation, the variation in the adjustment factor reduces sharply, and varies from 1.45 to 1.75 for R25 roundabout and from 1.5 to 2.3 for R37 roundabout The exponential rise in the adjustment factor with the increase in circulating flow changes to flatter but curvilinear function form when the estimation shifts from original HCM equation to adjusted HCM equation Some factious values of two wheelers, cars, autos and buses has been taken to see the variations of entry capacity values with respect to different flow conditions and varying proportions as given in Table The adjustment factors are extracted using figure and multiply to the adjusted HCM 2010 equation to estimate the entry capacity The estimated entry capacity with respect to circulating flow is shown in figure Table Estimated entry capacity using adjustment factor Adjustment Factor Estimated Entry Capacity (pcu/h)* TW (Veh/h) Circulating Volume Car (Veh/h) Auto (Veh/h) Bus (Veh/h) Total (pcu/h) R 25 R 37 R 25 R 37 500 450 150 41 1090 1.22 1.49 2262 2759 600 500 200 50 1290 1.26 1.54 2137 2606 650 550 250 45 1414 1.29 1.57 2063 2516 750 550 240 75 1563 1.32 1.61 1977 2411 850 750 204 57 1752 1.36 1.66 1873 2285 1000 800 280 60 1998 1.42 1.73 1747 2131 856 857 428 103 2215 1.47 1.79 1642 2003 1200 900 380 81 2407 1.51 1.84 1555 1897 1113 1142 428 77 2621 1.56 1.91 1463 1785 1280 1000 560 100 2800 1.61 1.96 1391 1696 1350 1125 430 155 3002 1.66 2.03 1313 1601 1512 1470 379 103 3271 1.73 2.12 1216 1483 * Adjusted HCM 2010 entry capacity 61 Estimated entry capacity (pcu/h) Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 3000 2500 y = 3761e-3E-04x R² = 2000 1500 y = 3083.7e-3E-04x R² = 1000 R25 R37 500 0 500 1000 1500 2000 2500 Circulating flow (pcu/h) 3000 3500 Figure Estimated entry capacity versus circulating flow Conclusions The estimation of entry capacity of any roundabout under conditions prevailing in developing countries is a tedious process because of heterogeneity in vehicle types having variation in their operational performance and the driver behavior, which is highly adaptable towards the prevailing traffic flow on the road rather than being influenced and governed by the geometrics and controls This paper tries to estimate the entry capacity of a roundabout based on the traffic flow conditions on a roundabout and drivers’ behavior depicted by the gaps being accepted or rejected while trying to enter the circulating flow It tries to come up with a simple approach which can be easily implemented by the practicing engineers in field Following points emerged from the analysis of the two roundabouts, one with diameter of central island as 25 m and other with 37.5 m The entry approach and circulating section are both classified as two-lane systems though there is some difference in the widths The two roundabouts catered to light traffic as majority of the composition constituted cars, motorized twowheelers and auto-rickshaws (motorized three-wheelers) This also got clear from the estimation of traffic in measurements units vehicle per hour and pcu per hour The trend of pcu/h traffic curve remained below the veh/h traffic curve The variation between the two increased with an increase in the circulating flow This probably indicated towards merging of small size vehicles more than cars or large size vehicles at higher traffic flows It further indicated that small size vehicles might be entering circulating flow by accepting smaller gaps which otherwise are rejected by larger size vehicles The entry capacity of the roundabout with larger diameter central island is found to be more than that of roundabout with smaller diameter As other geometrics especially number of lanes at entry and in circulating section are found equivalent, the only reason looks to be the increase in spacing between entry/exit locations which can accommodate more number of vehicles The relationship between entry capacity and circulating flow is found to be negative exponential i.e the entry capacity decreased exponentially with an increase in the circulating flow The use of original equation as given in HCM 2010 for the estimation of entry capacity will give quite low values in the traffic flow conditions prevailing in developing countries Two factors are identified behind this: one, traffic flow heterogeneity in developing countries as compared to the US and second, the driver behavior which is governed by lane discipline in the US but is governed by the flow characteristics and opportunities arising for merging The presence of about 40% two-wheelers in the traffic stream has led to higher value of entry flow as twowheelers show higher aggressive or impatient behavior while entering the circulating roadway as compared to 62 Srinath Mahesh et al / Transportation Research Procedia 17 (2016) 52 – 62 large size vehicles Therefore, the critical gap and follow-up time values as recommended by HCM 2010 are not applicable to traffic conditions in developing countries like India The follow-up-time is found to be lower than 0.60 times of critical gap as being reported in literature from mainly developed countries This too indicates that more number of vehicles can enter the stream in developing country traffic conditions as compared to traffic flows in developed countries This has direct influence on the entry capacity A value of 2.2 s for stream critical gap and 1.2 s for follow-up-time is suggested for estimation of entry flows at roundabouts in developing countries Adjustment factors are proposed for entry capacity estimation on roundabouts under two conditions, one when original HCM equation is used and other when it is adjusted with respect to the real values of critical gaps and follow-up-time prevailing in India Either of the two can be used by traffic engineers to compute the entry capacity of the roundabouts References Akcelik, R., Chung, E., and Besley, M 1998 Roundabouts: Capacity and performance analysis Research Report No 321, ARRB Transport Research Ltd, Australia Al-Masaeid, H., and Faddah, M 1997 “Capacity of roundabouts in Jordan Transportation Research Record: Journal of the Transportation Research Board, No 1572, 76–85 Bovy, H., Dietrich, K., and Harmann, A 1991 Guide Suisse des Giratoires Lausanne, Switzerland Brilon, W 1988 Recent developments in calculation methods for unsignalized intersections in West Germany Intersections without Traffic Signals, Springer Berlin Heidelberg, 111–153 Brilon, W., Wu, N., and Bondzio, L 1997 Unsignalized intersections in Germany-A state of the art Proceedings of the Third International Symposium on Intersections Without Traffic Signals, Portland, Oregon, USA, July 21-23, 61–70 Hagring, O., Rouphail, N M., and Sørensen, H A 2003 Comparison of capacity models for two-lane roundabouts Transportation Research Record: Journal of the Transportation Research Board, No 1852, 114–123 HCM 2010 Highway capacity manual 2010 Transportation Research Board, National Research Council IRC-65 1976 Recommendation practice for traffic rotaries Indian Roads Congress, New Delhi, India Kimber, R M 1980 The traffic capacity of roundabouts TRRL Laboratory, Report 942, Crowthorne, Berkshire, U.K Polus, A., Lazar, S., and Livneh, M 2003 Critical gap as a function of waiting time in determining roundabout capacity Journal of Transportation Engineering, ASCE, 129(5), 504–509 Polus, A., and Shmueli, S 1997 Analysis and evaluation of the capacity of roundabouts Transportation Research Record: Journal of the Transportation Research Board, No 1572, 99–104 Tian, Z., Troutbeck, R., and Kyte, M 2000 A further investigation on critical gap and follow-up time Transportation Research Circular E-C018: 4th International Symposium on Highway Capacity, Maui, Hawaii, June 27–July 1, 397–408 ... there is queue formation in the approach Queue formation represents the saturated condition of that approach The duration and composition of the queue is also recorded along with the corresponding... collection was done in the city of Chandigarh, capital city of both, the states of Punjab and Haryana Most of the intersections in the city are in the form of roundabouts having four approaches... from an approach and in circulating area in front of it are shown in Figure and for the two roundabouts respectively As mentioned before, the entry flows and circulating flows corresponds to the