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Efficient Traffic Control with Variable Speed Limits: Bridging the gap between Theory and Practical Implementation

Periodic Reporting for period 1 - ETC-VSL (Efficient Traffic Control with Variable Speed Limits: Bridging the gap between Theory and Practical Implementation)

Reporting period: 2017-02-01 to 2019-01-31

Traffic congestion on freeways is a critical problem due to its negative impact on the environment and many other important consequences like delays, waste of fuel, a higher accident risk probability, etc. Freeways were originally conceived to provide virtually unlimited mobility to road users. However, the continuous increase in car ownership and demand has led to a steady increase (in space and time) of recurrent and non-recurrent freeway congestion, particularly within and around metropolitan areas. The construction of new freeways is not always a viable solution that can be implemented in the short term due to technical, political, legal, or economic reasons. Therefore, in the last decades, a lot of research has been focused on making a better use of the available traffic infrastructure. It has been demonstrated in the literature that dynamic traffic control is an excellent solution to decrease congestion. In general, dynamic traffic control uses measurements of the traffic conditions over time and computes dynamic control signals to influence the behavior of the drivers and to generate a response in such a way that the performance of the network is improved, by reducing delays, emissions, fuel consumption, etc. Variable Speed Limits (VSL), ramp metering, and reversible lanes are some of the most often used examples of dynamic freeway traffic control measures. These measures have been already successfully implemented in Germany, Spain, The Netherlands, US, Australia, France and other countries.

Nowadays, most of the dynamic traffic control systems implemented operate according to linear, local or heuristic control algorithms. However, the use of appropriate non-local and multivariable techniques can improve considerably the reduction in the total time spent by the drivers and other traffic performance indices. Among the available options described in the literature, the methods based on the use of advanced control techniques like Model Predictive Control (MPC), which minimizes a cost function like the total time spent by the drivers, have shown to substantially improve the performance of the controlled traffic system in various simulation studies. The main problem of MPC is that the computation time quickly increases with the size of the network, making it difficult to apply centralized MPC for large networks. For this and other reasons, completely centralized control of large networks is viewed by most practitioners as impractical and unrealistic.
In this context, the main objective of ETC-VSL is the design and testing of a control algorithm for Variable Speed Limits (VSL) that approaches the behavior of an optimal controller and that can, at the same time, be applied in practice to large traffic networks.

The project has reached this objective by the proposal of the Logic-Based Traffic Flow Control algorithm (or LB-TFC). This control algorithm is able to control VSL (together with ramp metering (RM)) approaching the behavior and the performance of a centralized controller while its computation is almost instantaneous. This behavior has been tested in two networks (one synthetic network and a real stretch of the SE-30 freeway in Seville, Spain). The results show that LB-TFC provides a robust performance that, in most cases, is close to the optimal one and that significantly improves the reduction in the Total Time Spent (TTS) obtained with the Mainstream Traffic Flow Control (MTFC) + PI-ALINEA algorithm (MTFC + PI-ALINEA), which is the most well-known integrated controller for VSL and RM. Moreover, the tuning of the control parameters is studied for both freeways showing that the performance of LB-TFC is quite robust, especially when compared with the one obtained with MTFC + PI-ALINEA.

During the process of research, other interesting controllers have been proposed:
- SPEed limit controller for Recurrent Traffic jams (SPERT), which can properly control VSL in the case of recurrent congestion.
- Feed-Forward ALINEA (FF-ALINEA), which improve the performance of previously proposed controller for ramp metering if the congestion is created by a bottleneck located far away.

Moreover, a new model for the inclusion of VSL in macroscopic traffic models has been proposed based on the data from the A12 in The Netherlands.
The results have turned out in the publication of 5 papers (2 of them being currently under review) in the two best journals of the field (IEEE Transactions on Intelligent Transportation Systems and Transportation Research Part C: Emerging Technologies), in 4 presentations in some of the most prestigious international conferences on the field and many dissemination and public engagement activities (a presentation in the MSCA Falling Walls Lab contest in the context of the European Researchers' Night, 3 invited lectures on M.Sc. courses, publications in generalist newspapers…).
"The project has successfully contributed to the state of the art of freeway traffic control by reaching the following achievements:

- The proposal of a logic-based speed limit control algorithm for Variable Speed Limits to reduce traffic congestion at bottlenecks (LB-VSL).

- The proposal of an easy-to-implement logic-based traffic flow control algorithm for ramp metering (RM) and variable speed limits (LB-TFC).

- The proposal of a VSL controller for recurrent congestion based on an offline computed optimal solution (SPERT).

- The proposal of a new ramp metering control algorithm, Feed-Forward ALINEA (FF-ALINEA), for bottlenecks located both nearby on an on-ramp and further away from it.

- The proposal of a new macroscopic model for VSL that has the capability of modeling different capacities, critical densities, and levels of compliance for segments affected by speed limits.

- An analysis of the effects of VSL on the fundamental diagram of traffic flows, based on data from the A12 freeway in The Netherlands, concluding that the capacity of the freeway segment is decreased (and the critical density is increased) when the speed limit is reduced, that the VSL-induced fundamental diagram is not triangular and that the speed limit compliance can be very low if enforcement measures are not applied.

- The first identification, validation, and comparison of the most well-known macroscopic models for VSL.

The achievements of the project (especially the proposal of a control algorithm for VSL and RM installations that approaches the behavior of an optimal controller and that can, at the same time, be applied in practice to large traffic networks) may have an impact on European society, including the science base and the economy. Future implementations of the proposed controllers on real freeways would allow to significantly reduce traffic congestion, a fundamental step for reaching one of the societal challenges of Horizon 2020: “Smart, Green and Integrated Transport”. In fact, as stated in the European Commission's “White Paper on Transport: Roadmap to a Single European Transport Area - Towards a competitive and resource efficient transport system"" congestion costs are projected to increase by about 50% by 2050, to nearly 200 billion euros annually, if measures are not taken. This project will help to limit this increase by promoting the implementation in practice of the proposed algorithms for VSL and ramp metering installations.