International Journal of Scientific & Engineering Research, Volume 6, Issue 4, April-2015 219
ISSN 2229-5518
An Evaluation of the Capacity of two
Roundabouts: A Case Study of Agartala, Tripura
Amitabha Acharjee, Viki Das, Dr. Manish Pal
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HE second largest city in North-East India after Guwahati is Agartala. It is also the capital of Tripura. Agartala city is one of oldest city developed in 1844 AD. Agartala city is
connected with Bangladesh by Akhaura Road. Agartala city is connecting by the road to other part of state and the high pos- sibility of establishing of business corridor in Agartala. Traffic congesting is frequently observed in major intersection of Agartala.
Roundabouts are suitable when the traffic entering from three or more approaches are relatively equal. A total volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume of 500 vehicles per hour is the low- er limit. Roundabouts are suitable when there are more ap- proaches and no separate lanes are available for right-turn traffic thus making intersection geometrically complex. The traffic operations at a roundabouts are three; diverging, merg- ing and weaving [1].
Roundabouts have been widely accepted and used in countries for decades. A roundabout is a form of intersection design and control that accommodates traffic flow in one di- rection around a central island. Compared to uncontrolled intersection, roundabout can reduce speed as well as number of conflict points because of the geometrical design. The over- all delay will probably be less than that for signalized intersec- tion. Therefore, it is necessary to evaluate the feasibility of roundabouts as an intersection control alternative in the city of Agartala [2].
Significant study has been done to develop methodologies to evaluate the functional performance of roundabouts.
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• Amitabha Acharjee is currently pursuing Masters Degree program in Trans- portation Engineering in National Institute of Technology, Agartala, India, PH-+919774544399. E-mail: amitabhaacharjee1990@gmail.com
• Viki Das is currently pursuing Masters Degree program in Environmental
Engineering in National Institute of Technology, Agartala, India, PH-
+919436123382. E-mail: vikidas99@gmail.com
• Dr. Manish Pal is currently Associate Professor in Civil Engineering De-
partment in National Institute of Technology, Agartala, India, PH-
+919856024665, E-mail: mani_nita@yahoo.co.in
Significance of this study is to evaluate the capacity of roundabout as an intersection control alternative in Agartala. Roundabouts can be a good replacement for all-way stop con- trol where traffic volume are high, because priority is assigned to circulating traffic and yield-at-entry control allows vehicles to enter without stopping when gaps are available. Capacity models for the weaving sections using interweave theory model Wardrop (1973) formula is considered to be a typical representative of this model gap technique is applied in this paper. Capacity is a useful indicators used by most countries to assess roundabout performance [2].
Indian cities are facing the crisis of urban transportation. Despite of investments in road infrastructure and plans for transport development, users face the problem of congestion, accidents and pollution. Accident is a major problem, especial- ly at the intersection, as the vehicles on the intersection move at very high speed. Also due to traffic jam, lot of time is wast- ed. Pedestrians face troubles in crossing the road. Due to con- gestion, pollution increases and it causes harmful effects on human health living adjacent to the area. At any intersection delay and improper management as well as poor control over the flow of traffic increases rapidly. At intersection of Hari Ganga Basak road and VIP road, above problems frequently occurs due to heavy traffic flow [3].
The site has been so selected that within a stipulated time frame the maximum variety of data can be collected for the utilization in the study. Site could be selected in such a place where video graphic survey is to be conducted. In present study, two very important congestion areas had been selected near in Agartala. One has been selected in Post Office Chowmuhani and another one is selected in Circuit House Chowmuhani in Agartala.
Interweave theory model, gap acceptance theory model and regression analysis theory are the fundamental three method-
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International Journal of Scientific & Engineering Research, Volume 6, Issue 4, April-2015 220
ISSN 2229-5518
ologies can be used to assess the capacity of roundabouts. The typical representative for the interweave theory model is Wardrop (1973) formula. Based on the weaving theory, the model of capacity on weaving segment is established to reflect the capacity of roundabouts. A total of 2(Two) roundabouts were selected in Agartala city for data collection. The selected roundabouts should satisfy the following conditions in the selection of candidates.
A total of 2(Two) roundabouts were selected in Agartala city for data collection. The selected roundabouts should satis- fy the following conditions in the selection of candidates. First, there should be an area where a digital video camera can be mounted to tape the entire roundabout and the weaving sec- tion. Second, the selected roundabouts should meet the de- mand of large and heavy traffic, but with no congestion or any incident occurring at the roundabout.
The entry flow and the circulating flow of rotary intersection can be determined by using the radar gun. In Circuit house roundabout, the relationship between the speed of entry flow and circulating flow are illustrated below in fig. 1. It is evident that at weaving sections when the paths of two or more traffic streams cross in the same general direction, conflict and inter- ference increase rapidly. This situation results in a reduction of speed. By conducting significance test, the difference be- tween the speed of entry flow and circulating flow is statisti- cally significant. Therefore, it is necessary to study the weav- ing sections.
Circuit house roundabout having 4(Four) approach, One approach is towards Radha nagar, another three are towards airport Road, Krishna Nagar Road and G.B Road. Among them Radha nagar approach is one of the busiest one. The ve- hicles data collected from circuit house roundabout are tabu- lated in fig 2.
Present vehicle data like total vehicles, three wheeler, two wheeler, jeep, trucks in Agartala city has been collected from transport information center in Agartala. The collected data is used in regression analysis to develop suitable equation. Us- ing this equations vehicles growth percentage is determined. It was observed that for two wheeler, three wheeler and for four wheeler the growth percentage will be 96%, 88 % and 107 % respectively at 2022. The maximum capacity of Roundabout at Circuit House in 2022 by Wardrop formula is calculated as
3009 PCU/hour.
Similar procedure is followed in Post office chowmuhani
Roundabout and it is observed that its traffic capacity 3599
PCU/hour at present. Fig. 6 shows the PCU value in various
directions at Post office chowmuhani Roundabout. Fig. 7 and
Fig. 8 show the signal design at Post office chowmuhani
Roundabout.
The capacity of Roundabout at Circuit House as well as post office chowmuhani is calculated by Wardrop formula,
QP= (280w (1+e/w) (1-p/3))/ ((1+w/l))….. (1)
The capacity of rotary is directly determined by the capacity of each weaving section. The capacity of a weaving section is determined by the geometric layout, including entrances and exits, and the percentage of weaving traffic. The Transport and Road Research Laboratory (U.K.) which has pioneered re- search on this aspect, recommends the following formula, which is a modification of the well-known Wardrop formula (Equation 1).
Fig. 1. Relationship between the speed of entry flow and circu- lating flow in Circuit house roundabout.
Fig. 2. Showing the present PCU value in various directions at
Circuit House Rounabout.
Fig. 3. Geometric layout of Circuit House Roundabout.
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V1= the speed of vehicles which enter into the circulating flow, Km/hour.
V2= the speed of vehicles which drive out of the circulating flow, km/hour.
V3= the speed of entry flow, km/hour.
Fig. 4. PCU value in various directions at Circuit House Round- about at 2022.
Fig. 5. Speed vs. Time plot of Post office chowmuhani rounda- bout at Agartala.
Fig. 6. Showing PCU value in various directions at Post office chowmuhani Roundabout.
Fig. 7 and Fig. 8 show the signal design at Post office chowmu- hani Roundabout
Where,
Qp= Practical Capacity of the weaving section of the roundabout in PCUs per hour
w= with of the weaving section in meters.
e= average entry with of the roundabout in meters
l= Length of weaving section between the ends of the chan-
nelizing island in meters
P=Proportion of weaving traffic.
Roundabouts are suitable when the traffic entering from three or more approaches are relatively equal. A total Volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume of 500 Vehicles per hour are the lower limit as per IRC. Hence, Circuit house Roundabout is suitable for present condition, but as the study shows that it exceeds the upper value in the year 2022 and the Roundabout will be collapsed by that time.
In case of Post office chowmuhani Roundabout the total traffic at the junction is 3599 PCU/hour which is much more than the limit of 3000 PCU/hour specified by IRC. So, it can be con- cluded that the present condition is not sufficient for control- ling traffic operations at that Roundabout. For that most con- venient way to design the traffic is signal design which is giv- en a tried over here. In future scope of work it needs to be adopt other alternatives to accommodate the traffic.
The authors of this paper extend his sincere gratitude and thanks to Dipankar Sarkar, Assistant Professor, Civil Engi- neering Department and HoD, Civil Engineering Department of National Institute of Technology, Agartala for providing their valuable supports and cooperation.
[1] S.K Mahajana, A. Umadekarb, K. Jethwa, 2013. New Concept of Traffic Rota- ry Design at Road Intersections. Procedia - Social and Behavioral Sciences. In- telligent and Integrated Sustainable Multimodal Transportation Systems Pro- ceedings from the 13th COTA International Conference of Transportation Professionals (CICTP2013). Volume 96, 6 November 2013, Pages 2791–2799. doi: 10.1016/j.sbspro.2013.08.312.
[2] W.Wang, X. Yang, 2012. Research on Capacity of Roundabouts in Beijing. Procedia - Social and Behavioral Sciences. 8th International Conference on Traffic and Transportation Studies (ICTTS 2012). Vol- ume 43, 2012, Pages 157–168. doi: 10.1016/j.sbspro.2012.04.088.
[3] M.Patel, (2014). Solution for reduction of traffic congestion. Interna- tional Journal of Applied Engineering and Technology. Vol. 4 (1) January-March 2014, pp.37-45/Patel.
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