Improving the ramp metering performance in traffic networks by personal in-car route advice

Abstract

In the Netherlands and all over the world congestion is still an everyday occurrence. In order to reduce this congestion the capacity of the freeways is increased and dynamic traffic management measures (DTM) are applied. One of these DTMs is ramp metering. Ramp metering systems decrease the inflow to the freeway in order to prevent the demand from exceeding the capacity of the freeway. However, the time that a ramp metering system can be active is limited. The use of navigation systems can also help reduce congestion. More and more road users have the possibility to use navigation systems in their car. These navigation systems are currently only designed to find the fastest route to a destination. But the possibilities for data exchange with these systems are increasing. A very interesting subject of study will be to examine the possibilities of combining the world of DTM and navigation systems in order to optimize the performance of the network which would lead to a reduction in vehicle loss hours. This issue takes a central place in the research in this report. The main question is to develop a routing control system that is able to improve the network performance by better utilizing the storage capacity of the on-ramps by rerouting road users at the secondary road network. This research is undertaken by first analysing the problem of limited storage space from a traffic management and control engineering point of view. This is followed by the development of a routing control system that should improve the network performance and finally the network performance is tested in an example model to verify the expectations. From a traffic management point of view the main problem is an exceeding of the flow capacity of the freeway. This is due to the sum of the flow on the freeway and the flow of the on-ramp being greater than the freeway’s capacity. As an intervention, the ramp flow is limited by the ramp metering system, but this can only be done very briefly, due to the limitation in the available storage space at the on-ramp (to prevent spill back onto the secondary road network). To increase this time, the storage capacity can be increased by a physical increase of the length of the on-ramp, the flow of the freeway can be decreased (by coordination of upstream on-ramps for example) or the demand of the on-ramp can be decreased. This decrease of the ramp demand could theoretically be obtained by rerouting traffic by the use of data of the ramp metering systems, in in-car navigation systems as is the subject of this study. To be able to reroute drivers a control system has to be set. The target of this control system is to improve the network performance by giving some alternative route advice. To achieve a realistic alternative some information of the driver is needed (current location and route to his destination) and information of the network should be used (network topology and actual flows and speeds of the various roads). Based on this information the controller itself can be elaborated. In this controller the following four steps can be distinguished: - Determination of problems at the various on-ramps In order to be able to choose between rerouting traffic or not, an estimation of the potential problem at the on-ramp needs to be made. To be able to come to a good approximation of the network situation, a prediction model must be used. From this prediction it can be determined if there is a problem at the on-ramp. There is a problem if the size of the queue exceeds the critical queue length. This critical queue length is a threshold level that is some vehicles below the maximum. - Select vehicles that contribute to that problem Based on the determination of the problem, all vehicles (with a navigation system) that contribute to this problem can be selected in the network. - Search for realistic route alternatives An alternative route needs to be searched for the vehicles that contribute to the problem. This alternative route needs to comply with two conditions: o Additional travel time with respect to original route should be below the threshold difference (for example 5 minutes) o No problems at the alternative route (queue length at possible other on-ramp also below the critical threshold level) should occur - Reroute vehicles If all the conditions, mentioned above, are complied with, vehicles will be rerouted to prevent the queue at the on-ramp from exceeding the critical queue length. Finally the performance of the control system is tested in an evaluation study. The routing control system has improved the network performance at the various scenarios. As expected the storage capacity of the various on-ramps was better utilized and that results in a delayed occurrence of the queue at the freeway. The size of the improvements depends greatly on the availability of realistic route alternatives. A network is needed with various routes to the freeway with a small difference in the travel times between the various routes. The performance also depends on the distribution of the traffic over the various routes and especially the on-ramps. The greater the differences in the ramp demands, the worse the coordination of the on-ramps become to achieving the optimal result itself. This is due to the limitation in the minimal ramp flow and the variations in the size of the storage capacities at the various on-ramps. Besides the network performance the travel time reliability has also improved. The travel time reliability is the variance in the travel time over a certain route within the model period. The reliability is better, since the travel times vary to a lesser degree and the maximum travel time is decreased. Due to the largest variation in the travel time of vehicles at the freeway (because of the queue at the freeway) the improvement in the reliability of the travel time of road users that do not have to pass an on-ramp is the greatest. It can be recommended to further research this topic. The scope of this study is limited to rerouting traffic from the main on-ramp under the conditions that have been mentioned above. Maybe other road users can also be rerouted (traffic freeway, road users with an upstream on-ramp on their route) to improve the network performance. Driver behaviour (acceptance of additional travel time, proportion of drivers with access to a navigation system, willingness to follow of alternative route advice) is also a potential topic for further investigation. Besides that an additional evaluation can be done to obtain extensive conclusions and the possibilities to implement the system in practise should be researched further.