University of Twente Student Theses


Integrated Traffic Control with Variable Speed Limits and Coordinated Ramp Metering based on Traffic Stability

Eikenbroek, O.A.L. (2013) Integrated Traffic Control with Variable Speed Limits and Coordinated Ramp Metering based on Traffic Stability.

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Abstract:For the last decades, congestion on highways has been a serious issue. Congestion causes more air pollution, accidents and delays, resulting in late arrival for employment or meetings. One of the main goals of the field of transportation engineering is to reduce or prevent delays and accidents caused by congestion, although, the last years it has been becoming more apparent that congestion cannot be completely solved. The main reason is simple; the highways are carrying too much vehicle miles, more than for it was designed At first sight, an easy way to resolve congestion is the construction of new roads. By constructing new roads the capacity of the highway system will be higher and the number of traffic jams will be reduced, but is an expensive solution. Innovative solutions such as ramp metering and variable speed limits can be used to reduce congestion on a highway. In order to resolve or prevent congestion essential is the predictability of congestion. Congestion is a result of a combination of three ingredients; a high traffic volume, a local perturbation and a spatial inhomogenity. Studies by Lee et al. (2000) and Treiber et al. (2000) have shown that the relation between these three ingredients can be predicted by traffic stability. The influence of a local perturbation (e.g. a lane change) on the flow can eventually cause congestion if the traffic flow is unstable. This validates that it is essential to measure the stability of traffic flow. Two discussed stability indicators are able to determine the stability of traffic in free flow. A stochastic approach is able to determine whether traffic has entered the metastability regime (only large perturbations lead to congestion) and a reliability indicator, based on the variance in flow, is able to determine the stability. To make sure congestion will not arise, traffic can be controlled. Two control strategies, ramp metering and variable speed limits, are discussed. Ramp metering strategies control the amount of vehicles entering the mainline. Literature has shown that local and coordinated ramp metering strategies are able to improve the efficiency of a highway. Variable speed limits can be used in order to homogenize traffic or to prevent traffic breakdown by limiting the inflow of a highway segment. The homogenization approach is, based on the definition of traffic stability, causing a lower capacity. The more complex approach, preventing breakdown, is able to reduce travel time as simulation studies have shown an improvement of travel time up to 50 % (Lu et al., 2011; Su et al., 2011). As it was shown that ramp metering strategies and variable speed limits are able to reduce travel time and stability indicators are able to predict congestion, these two are combined for new control strategies. Based on a local ramp metering strategy, a correction factor is proposed based on traffic stability. If traffic flow is unstable fewer vehicles should be allowed to enter the highway, if traffic stable the correction factor is high. Further research is needed to determine the performance and the influence of infrastructure layout and flow characteristics on the correction factor. An integrated control strategy, where the variable speed limit is determined before the ramp meter rate, is proposed based on the preventing breakdown approach. As the detector measurements show that traffic is getting unstable, the control is switched on. Here, the speed limit on a highway segment is based on the difference between the desired and the measured occupancy. After the variable speed limits are set, a model predictive control scheme is used to find the optimal ramp meter rate. Every detector interval (30 seconds) the control scheme minimizes the total travel time and maximizes the total traveled distance. These calculations are used for implementation. The proposed strategies are implemented in the simulation environment in order to test their performance. The network, modeled in the microsimulation software Aimsun, is a part of the I-880 NB in California, USA. Implemented in the simulation software is the section from post mile 9 (Dixon Landing Road, Milpitas) to post mile 18.66 (Central Ave, Fremont). Five different strategies are compared; the uncontrolled case, the local ramp metering strategy ALINEA, the proposed variable speed limit strategy, the proposed coordinated ramp metering strategy and the proposed integrated strategy. Results show that the integrated strategy is best performing, improving the total time spent with 6 %. Other control strategies show less improvement and the ALINEA strategy shows even less performance. The results are in line with literature but show less performance, expectedi s that the short on-ramps are highly influencing the performance. Concluded is that the control strategy is able to resolve the congestion earlier. Before considering application in real world, variable message signs are needed. Besides, the performance could be improved by increasing the prediction horizon or considering an extra constraint based on the reliability indicator. Essential is, testing the proposed strategy in a simulation model on several days.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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