Transportation Research Part A 37 (2003) 681–688 www.elsevier.com/locate/tra
Dynamic vehicular delay comparison between a police-controlled roundabout and a traffic signal Hashim M.N. Al-Madani
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Department of Civil & Architectural Engineering, University of Bahrain, P.O. Box 32038, Bahrain Received 20 March 2000; received in revised form 28 April 2002; accepted 22 February 2003
Abstract Effects of queues on motorists during rush hours are severe at intersections controlled by roundabouts. Traffic police are frequently used in order to optimize the traffic flow and to control queue length at such intersections. However, the question as to how efficient such system is, compared with traffic signal, is not clear from the dynamic delay point of view. In this study a criterion is being developed based on vehicular delays as the motorist join the queues and cross the stop-line. The adopted method avoids oversimplification of reality and prevents unrealistic assumptions. The data required for the study were mainly collected through video filming technique. The results, for a given set of geometric and traffic characteristics, indicate that both a police-controlled roundabout and a traffic signal act in a similar manner in terms of vehicular delay at a certain critical value. This critical value is considered to be the point of intersection between the curves representing traffic signal and roundabout on a delay–space diagram for the vehicles as they join the tail end of the queue until they cross the stop-line. Beyond the critical value, the effect of delays and buildup of queues at roundabouts will be excessive, compared to traffic signals. Before the critical value the delays at traffic signals are quite high compared to roundabouts. The study will assist the concerned authorities to operate the existing conditions, particularly the roundabouts, more efficiently. It will also be beneficial for the traffic planners and policy makers in making judicious decisions regarding control type at intersections. Ó 2003 Elsevier Ltd. All rights reserved.
1. Introduction Queues build up excessively during peak periods on all approaches of the major intersections. The effect of such queues on motorists are severe at intersections controlled by roundabouts. In
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order to direct the traffic at roundabouts more smoothly, to give better chances for mass crossings of traffic, and to have better control on queue length it is necessary to have the traffic on such intersections controlled by police. Otherwise, motorists will face excessive uncontrolled delays. However, the question as to how efficient such system is compared to isolated signalized intersections, from dynamic vehicular delay point of view, needs to be investigated further. In other words, a clear criterion between the two control systems, based on vehicular delays as the motorists join the queue until they cross the stop-line, is required in the decision making process. Dynamic vehicular delay at intersections is a major current concern, because the standard static network equilibrium formulation fails to capture essential features of traffic congestion (Mahmassani and Michalopoulos, 1985; Ben-Akiva, 1985). During peak periods the optimal solution is to minimize the intersection delays subject to queue length (Stephanopoulos et al., 1979; AlMadani, 1992a,b). Numerous studies related to dynamic traffic modeling for congested signalized intersections have been carried out both deterministically, where the randomness nature of traffic arrivals and departures are ignored (Kimber and Hollis, 1979; Hurdle, 1984; May, 1990), and stochastically, where the probabilistic characteristics of queue discharge are calibrated for varying conditions (Drew, 1968; Leutzbach, 1988). However, while application of shock wave theory at signalized intersections were covered extensively, very few dynamic studies have been made for police-controlled roundabouts, probably due to their limited numbers at major junctions compared to traffic signals. However, roundabouts have been more popular in erstwhile British Colonial countries. A basic application of shock wave theory at traffic signals indicates that vehicle delays will decrease as the location where a vehicle joins the queue moves further upstream provided that the vehicle is not stopped twice. This is the result of the stopping wave, as the red signal begins, intersecting with the starting wave, as the green light starts (Al-Madani, 1992a,b).
2. Objective This study deals with dynamic delay comparisons between police-controlled roundabouts and pre-timed traffic signals. The main objective is to establish a relationship between vehicular delay, as the vehicle joins the queue until it leaves the intersection, and vehicle location as it joins the queue. Consequently performance curves based on delay time and queue length are established. For each type of intersection control. Secondary objective, in terms of vehicular delays, is to test the validity of two major hypotheses. Firstly, roundabouts perform better than pre-timed traffic signals at intersections with moderate queue lengths. Secondly, as the queues build up excessively pre-timed traffic signals perform more efficiently than roundabouts even if the later is to be fully controlled by traffic police to optimize the traffic flow operation.
3. Methodology Vehicular delay estimation using an input–output process is employed at points where vehicles join a queue and depart the stop-line. The difference between the two times and locations, i.e. where the vehicles joined a queue and crossed the stop-time, are simply the travel time and the
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queuing distance. Subsequently, the minimum travel time at posted speed limit is subtracted to estimate the vehicle delay. Delays are obtained in such a way so as to express the condition more realistically both microscopically, since individual drivers count the delay from the moment they are impeded by the traffic ahead, and macroscopically, since delays of a group of vehicles in a queue will be a key element in dynamic modeling and in queue management (Michalopoulos and Stephanopoulos, 1977; May et al., 1987). The method applied here avoids oversimplification of reality and unrealistic assumptions. Times and space locations of vehicles joining a queue (tJ , dJ ) and leaving the stop-line (tL , dL ¼ 0) are required to establish the necessary curves. As can been seen from the simple time– space diagram (Fig. 1) of vehicle trajectories, the vehicles are observed as they join and dissipate from a queue at a gated intersection either by a traffic signal or by police. Vehicles times, tJ0 to tJ4 , are recorded as each vehicle joins the propagating queue at pre-selected stations (d0 to d4 ). The times of these vehicles as they cross the stop-line (tL0 to tL4 ) are to be recorded, as well. The stopped time delay while the vehicle is in the queue (STDQ) and vehicular delay time (VDT) can be determined from the following equations: STDQ ¼ tL tJ
ð1Þ
FVDT ¼ dJ =uD
ð2Þ
ðVDTÞ ¼ STDQ FVDT
ð3Þ
where, tJ is the time as the observed vehicle joins the queue at the corresponding station, tL is the time as it crosses the stop-line ðd0 Þ, dJ is the distance from the stop-line for the considered station, uD is the posted speed limit, and FVDT is the free-flowing vehicle dissipating time from the intersection. Acceleration and deceleration delay times are ignored since they are negligible compared to STDQ. While the traffic signal was set for optimum operation, the roundabout was assumed to operate at optimum condition when it is controlled by police. The traffic police controlling the roundabout clears one approach at a time. Although, the police will usually be clearing the approaches in order and with unequal intervals as required, yet frequently the police interrupts the order sequence to clear an adversely affected approach, where queues are approaching
Fig. 1. Time–space diagram.
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downstream junction or so. Other approaches will be blocked gradually, while the approach traffic is to be cleared, by the jammed circulating traffic in the roundabout.
4. Data acquisition The required data were collected through video filming technique using frame by frame facility. Accuracy of
