Active traffic management in metropolitan freeways : modeling and assessing dynamic speed limit strategies

Abstract

Traffic congestion is a major source of inefficiencies in metropolitan freeways (e.g. productivity loss, energy waste, pollutant emissions, among other externalities). Active traffic management (ATM) strategies are used in order to face the problem and alleviate its effects by managing recurrent and non-recurrent congestion with a combination of real-time and predictive operational strategies. Dynamic speed limit management (DSL) is an ATM tool believed to bring some benefits to freeway traffic, like the increase in capacity and/or the vehicle' speed homogenization. Currently it is being implemented in many metropolitan freeways worldwide. This thesis is devoted to assess such conceptual benefits with empirical data and robust traffic flow models. Empirical data are obtained from the C-32 freeway in Barcelona, the first DSL implementation in Spain. The detailed analysis of these data allows characterizing the effects of DSL management on traffic behavior. It is found that, under the right conditions, DSL can increase capacity due to a reduction in the speed dispersion across lanes. This finding allows defining new ATM policies. A coordinated strategy for traffic management considering both ramp metering and DSL is proposed in order to reduce the capacity drop phenomenon in the vicinity of a freeway on-ramp. The effectiveness of this control method is tested using simulation with an extension of the traditional cell transmission model, which incorporates the ability to reproduce DSL, together with the capacity drop. Results show amelioration in the performance indicators of the system, highlighting the equity-friendly component of this coordinated strategy. Finally, results from a cost-benefit assessment of DSL considering the main externalities suggest that the social profitability of DSL management in metropolitan freeways is limited when applied alone. The potential synergies of applying a ¿pack¿ of different ATM strategies in a coordinated way, define challenging issues for further research.