DOI: 10.25549/usctheses-c127-651456
title: Dynamic variable speed limit control: design, analysis and benefits

description: Variable speed limit (VSL) systems, as one of the freeway control strategies among Intelligent Transportation Systems (ITS), have been studied since 1970s. However, the cases of field implementations are limited, mostly in Europe and the United States. The VSL systems are considered to reduce risk of crashes, to warn drivers of hazardous roadway conditions, to stabilize and smooth traffic flows, to dampen shock waves, to postpone or prevent congestions, and to reduce emissions and fuel consumptions. The safety benefits of using VSL have been reported in several field studies both in Europe and the United States. Currently, the incentive of using VSL has been mainly safety from the application point of view. However, benefits such as improved traffic flow rates, lower travel times, smooth speed and density distribution and possibly lower pollution have been conjectured in literature and in some cases analyzed using mainly macroscopic traffic models. A closer look at these promising results in the literature shows that using the same macroscopic model to design and test raises questions as to whether the simplicity of the models used for evaluation are responsible for these optimistic results given the fact that other studies using microscopic simulations fail to demonstrate improvements on travel times albeit for different VSL strategies. The question whether the VSL strategies or the macroscopic models used to analyze them or both are responsible for the large differences in traffic flow and travel time benefits reported, remains unanswered. Consequently, the problem of what are the most appropriate dynamic VSL controllers and what benefits can be guaranteed in a consistent manner under different traffic flow conditions is an open one. ❧ In this study, the problems of the design, analysis and evaluation of dynamic VSL controllers are addressed. A control engineering approach is followed, where the control strategies are designed based on simplified models (in this case validated macroscopic traffic flow models) but applied and tested on validated microscopic traffic models under different traffic conditions. Three dynamic VSL control designs, a simple virtual metering strategy which is reactive and non-model based, a nonlinear model predictive controller which is proactive and model based, and a proportional speed controller which only needs traffic density/occupancy as inputs, are presented, tested using both macroscopic and microscopic simulation models. Monte Carlo simulations are conducted for 10 different scenarios using an integrated simulation/evalution framework. ❧ Examining the performance measurements summarized from several hundreds simulations runs, the simple PI type controller with less computational burden is not inferior to the more complicated nonlinear MPC which also needs predicted demand and accident information as inputs. Although macroscopic simulations demonstrates that both simple controller and model predictive controller could reduced Total Time Spent (TTS) for about 20\%, VISSIM microscopic simulations show that Total Travel Time (TTT) could not be improved by variable speed limit controllers due to the vehicle level transient responses and the second rate shock wave generated by slowing down traffic in advance both of which are not captured in macroscopic models. Safety benefits of VSL controllers such as reducing number of stops and reducing number of lane changes are demonstrated through VISSIM simulations. However, in order to get environmental benefits, VSL should be implemented in a way to force smoother speed profiles of individual vehicles. Simulation results also show that the effectiveness of VSL controllers are dependent on the traffic demand level and the congestion level. VSL controllers are more effective when traffic density is close to the critical density.