The claimed benefits of dynamic speed limit (DSL) strategies (e.g. increase of throughput or speed homogenization) still remain as a controversial topic. The present paper presents an empirical assessment of DSL policies with aggregated traffic flow data. A DSL system installed on the C-32 metropolitan freeway accessing the city of Barcelona (Spain) since February 2009 allows a before-after comparison between non-DSL and DSL scenarios. In order to achieve this objective, a new methodology is proposed. It includes speed-based data stratification for assessing the DSL drivers’ compliance, detecting the causes which motivate drivers’ behavior and the main DSL algorithms inefficiencies. In addition, a computation-friendly approach for searching stationary traffic periods is developed, differing from Cassidy (1998). The whole process permits to obtain a clear fundamental diagram characterization under DSL strategies. The paper proves that when speed limits are lower than free-flow speed, the slope of the free-flow branch of the fundamental diagram is lowered proportionally. This implies that critical occupancies are higher than in their non-DSL analogues. Remarkably, almost any capacity increase is reported in the vicinity of the analyzed bottleneck.
A coordinated strategy, composed of ramp metering together with dynamic speed limit management (DSL) is proposed in order to reduce the capacity drop effects in the vicinity of a freeway on-ramp. The strategy is based in a simple formulation which makes use of the mainstream metering capabilities of DSL and considers the resulting improvement of traffic flow homogeneity, in order to capture all the available merge capacity. The effectiveness of this new control strategy is tested via simulation. An extension of the cell transmission model, which incorporates the ability of reproducing DSL strategies and the capacity drop phenomenon is proposed. The simulation allows comparing different controlled/uncontrolled scenarios on a hypothetical freeway section containing an on-ramp. Results show amelioration in the performance indicators of the system. The equity-friendly component of this coordinated freeway strategy is also highlighted.
Some of the most frequently claimed dynamic speed limits (DSL) benefits (e.g. increase of throughput or speed homogenization) still remain a controversial topic among the scientific community, practitioners and society in general. The objective of the paper is to present an empirical assessment of this policy, aiming to provide an insightful response to the main issues related with the aggregated traffic flow behavior. Thanks to the DSL equipments installed over a metropolitan freeway accessing to Barcelona since February 2009, a comparison between non-DSL scenarios (dated on October 2008) and DSL-affected can be performed. For this purpose, a new methodology
has been proposed. The whole process excels in being simple, traffic flow theory consistent, achievable with common available traffic data and easily integrable in a programming language. In doing so, a reliable characterization of the traffic flow behavior under DSL
strategies has been obtained. Revealing results concerning to the driver’s behavior when facing different speed limits were obtained. The fundamental diagram calibration shows no capacity increments under DSL together with a significant critical occupancy value shift,
when are compared with the non-DSL case. It is also reported certain vehicle speed and lane use (i.e. occupancy) homogenization over the DSL controlled sections.
Dynamic speed limits (DSL) strategies are expected to delay onset of congestion thanks to its ability to improve traffic flow homogenization and its mainstream metering feature. A new coordinated strategy, i.e. ramp metering (RM) together with DSL, is proposed to reduce the capacity drop occurrence in the vicinity of an on-ramp. It is based in a simple formulation which captures the endogenous merge capacity. The effectiveness of the new control strategy is tested with a cell transmission model extension, which incorporates the ability to reproduce DSL strategies together with capacity drop phenomena. The simulation allows comparing different control/no-control scenarios considering a hypothetical freeway section containing an on-ramp. Results show amelioration in the performance indicators of the system, highlighting the equity friendly component of the coordinated freeway strategies.
The Asymmetric Cell Transmission Model (ACTM) is a widely accepted macroscopic model for freeway traffic. Essentially, the model is based on the original Cell Transmission Model (CTM), with modified merge equations in order to make it suitable for ramp metering optimization. However, an unfeasible merging behavior is observed when uncontrolled merges are simulated if using a simplistic but common calibration. A simple methodology for calibrating the two parameters involved in the ACTM merge model is proposed in the present paper. The method ensures the physical consistency of the dynamics involved in queue processes in the proximity of a non-controlled merge junction. This should be an essential feature of any freeway traffic model.
Many first-order macroscopic models for freeway traffic have been developed since the seminal work on the well-known Lighthill-Whitham-Richards, or LWR, continuous model. The asymmetric cell transmission model (ACTM) is a widely accepted macroscopic model integrated into a variety of freeway traffic control frameworks. The model is based on the original cell transmission model; modified merge equations make it computationally tractable for optimization of ramp metering. However, when the common simplistic calibration of the model is used, an infeasible merging behavior is observed at uncontrolled merges, and the resulting queue lengths are not accurately estimated. This paper proposes an elegant analytical relationship, based on traffic flow theory, for calibrating the two parameters involved in the ACTM merge model. The method ensures the physical consistency of the dynamics involved in queue processes in the proximity of a noncontrolled merge junction. This method should be an essential feature of any freeway traffic model.
La congestió del trànsit és la principal font d¿ineficiències en les autopistes metropolitanes (pèrdua de productivitat, d'energia o emissió de gasos contaminants, entre d'altres externalitats). Les estratègies de gestió activa del trànsit (ATM, en anglès) s¿utilitzen amb la finalitat d¿afrontar el problema i alleujar els seus efectes, mitjançant la gestió de la congestió, tant recurrent com no recurrent, amb una combinació d¿estratègies operacionals en temps real i de predicció.La gestió dinàmica del límit de velocitat (DSL, en anglès) és una eina d¿ATM que pretén aportar certs beneficis al trànsit de les autopistes, com l¿increment de la capacitat i/o l¿homogeneïtzació de les velocitats dels vehicles. Actualment, està implantat en moltes autopistes metropolitanes d¿arreu del món. Aquesta tesi està dedicada a avaluar aquests avantatges conceptuals amb dades empíriques i models contrastats de flux de trànsit.Les dades empíriques s¿obtenen de l¿autopista C-32 a Barcelona, la primera instal·lació de DSL a Espanya. L¿anàlisi detallat d'aquestes dades permet la caracterització dels efectes de l¿estratègia de DSL en el comportament del trànsit. S'ha trobat que, sota les condicions adequades, el DSL pot augmentar la capacitat de les autopistes a causa d'una reducció en la dispersió de les velocitats entre carrils. Aquesta contribució permet definir noves polítiques d¿ATM.Es proposa una estratègia coordinada per a la gestió del trànsit que té en compte tant les estratègies de control d¿accessos com de DSL, per tal de reduir el fenomen de la pèrdua de capacitat en les proximitats d¿una rampa d¿accés en autopista. L¿eficàcia d¿aquesta estratègia de gestió s¿avalua amb simulacions, mitjançant una extensió del model tradicional de transmissió per cel·les, que incorpora la capacitat de reproduir estratègies DSL, juntament amb la pèrdua de capacitat. Els resultats mostren una millora en els indicadors de rendiment del sistema, destacant els beneficis en termes d¿equitat que l¿estratègia coordinada aporta .Finalment, els resultats d¿un anàlisi cost-benefici de l¿estratègia DSL, considerant els principals factors externs, suggereixen que la rendibilitat social d'aquesta estratègia en autopistes metropolitanes, quan s'aplica de forma aïllada, és limitada. Les potencials sinergies derivades de l¿aplicació coordinada d¿un conjunt d¿estratègies ATM, defineixen un interessant repte per a futures investigacions.
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.
Soriguera, F.; Torne, J.; Rosas, D. Journal of intelligent transportation systems: technology, planning, and operations Vol. 17, num. 1, p. 78-90 DOI: 10.1080/15472450.2012.719455 Data de publicació: 2013-01 Article en revista
In July 2007, the Autonomous Government of Catalonia passed a 73-measure plan to improve the air quality in the metropolitan region of Barcelona, Spain. This plan (which turned into law in January 2008) included 80 km/h speed limitations on major highways around the city (e.g., the C-32 highway). One year later, dynamic speed limits (DSL), maintaining the maximum speed limit of 80 km/h, became operational. The objective of the present paper is the evaluation of this policy, what requires a quantitative analysis in terms of congestion reduction, vehicles‟ emissions and traffic safety.
As the comparison between different scenarios must be performed with homogeneous traffic demands (i.e., same productivity), comparisons from direct empirical measurements are not representative. Note that during the period of analysis, the huge economic recession arrived to Spain resulting in a clear reduction of mobility demands. CTMSIM macroscopic traffic simulator [1, 2] was applied instead, simulating different scenarios under the same demand. Data in order to construct the baseline demand scenario were collected from double loop detectors installed in each lane along 14.5km of the C32 highway, accessing Barcelona. A quantitative evaluation of the social profitability of this policy in terms of travel times, emissions and safety benefits is provided in the paper. In addition, some guidelines for the construction of homogenous demand scenarios suitable for comparisons, the characterization of fundamental diagrams, the macroscopic model of the highway based on the cell transmission model (CTM) [3, 4], and the definition and assessment of the different elements constituting the objective function in monetary units, are also described. It is concluded from the discussion, that the speed limit management on metropolitan highways accessing to large cities, cannot provide in case of isolated application significant benefits.