The vertex k-center problem is a classical NP-Hard optimization problem with application to Facility Location and Clustering among others. This problem consists in finding a subset <inline-formula> ...<tex-math notation="LaTeX">C \subseteq V </tex-math></inline-formula> of an input graph <inline-formula> <tex-math notation="LaTeX">G=(V,E) </tex-math></inline-formula>, such that the distance from the farthest vertex in <inline-formula> <tex-math notation="LaTeX">V </tex-math></inline-formula> to its nearest center in <inline-formula> <tex-math notation="LaTeX">C </tex-math></inline-formula> is minimized, where <inline-formula> <tex-math notation="LaTeX">|C| \le k </tex-math></inline-formula>, with <inline-formula> <tex-math notation="LaTeX">k \in Z^+ </tex-math></inline-formula> as part of the input. Many heuristics, metaheuristics, approximation algorithms, and exact algorithms have been developed for this problem. This paper presents an analytical study and experimental evaluation of the most representative approximation algorithms for the vertex k-center problem. For each of the algorithms under consideration and using a common notation, we present proofs of their corresponding approximation guarantees as well as examples of tight instances of such approximation bounds, including a novel tight example for a 3-approximation algorithm. Lastly, we present the results of extensive experiments performed over de facto benchmark data sets for the problem which includes instances of up to 71009 vertices.
Bad air quality due to free pollutants such as particulate matter (PM), carbon dioxide (CO 2 ), nitrogen oxides (NO x ) and volatile organic components (VOC) increases the risk of long- term health ...diseases. The impact of traffic-calming measures on air quality has been studied using specialized equipment at control sites or mounted on cars to monitor pollutants levels. However, this approach suffers from a large number of variables on the experiments such as vehicles types, number of monitored vehicles, driver’s behavior, traffic density, time of the day, elapsed monitoring time, road conditions and weather. In this work, we use a cellular automata and an instantaneous traffic emissions model to capture the effect of speed humps on traffic flow and on the generation of CO 2 , NO x , VOC and PM pollutants. This approach allows us to study and characterize the effect of many speed humps on a single lane. We found that speed humps significantly promote the generation of pollutants when the number of vehicles on a lane is low. Our results may provide insight into urban planning strategies to reduce the generation of traffic emissions and lower the risk of long-term health diseases.
In this paper we numerically study the probability Pac of the occurrence of car accidents in the extended Nagel–Schreckenberg (NS) model in the case of mixture of fast (Vmax1=5) and slow vehicles ...(Vmax2=1) by taking also to the risky overtaking of fast vehicles. In comparison with previous existing models, we find that accidents can occur in the free traffic phase and/or congested one depending on the overtaking rate of fast vehicles. The effect of evacuation of damaged vehicles from the road with probabilities Pevf and Pevs of fast and slow vehicles respectively on the traffic flow behavior is also computed.
•Realistic car accident model.•Risky overtaking of fast vehicles.•Occurrence of accident even in free flow traffic.
Unsignalized mid-block raised crosswalks have been adopted as inclusive transport strategies, providing humps to reduce vehicles’ speed to promote drivers to yield to pedestrians. The interaction ...between vehicles and pedestrians can induce local jams that can merge to become a gridlock. The purpose of this paper is to investigate the interaction between vehicles and the mid-block raised crosswalk, analyzing its effects on traffic flow, instantaneous CO2 emissions, and energy dissipation. A pedestrian–vehicle cellular automata model was developed, where a single-lane road with a mid-block raised crosswalk is considered. The lane boundaries were controlled with the injections rate (α) and extraction rate (β), while the pedestrians’ entrance was controlled with the rate (αp). The system’s phase diagram was constructed, identifying four phases: maximum current, jamming, congestion, and gridlock. All observed phase transitions are of the second order. The transition from maximum current (or jamming) phase to gridlock phase is not noticed. Moreover, since the crosswalk is a bottleneck, the gridlock phase takes place when the pedestrians’ influx exceeds a critical value (αp > 0.8). The study also revealed that the crosswalk is the main precursor of energy dissipation and CO2 emissions, whose major effects are observed during the jamming phase.
Unsignalized mid-block crosswalks are adopted to regulate unauthorized crossings by promoting drivers’ yielding to pedestrians. Although pedestrian–vehicle interactions at crosswalks have been widely ...studied, the effects of such facilities regarding traffic flow and emissions have been overlooked. This paper presents a vehicle–pedestrian cellular automata model to analyze the effects of two types of crosswalk systems: raised and zebra, on traffic characteristics. The model allows microscopically studying the impact of the induced speed heterogeneity in terms of traffic flow, CO2 emissions, and kinetic energy dissipation. Simulation results show that traffic flow experiences four phases in both systems: maximum current, congestion, jamming, and blocking; with second-order transitions among them. The maximum current phase is the only one with free-flow conditions shrinking faster in the system with a raised crosswalk than in the one with zebra. Both systems reach the blocking phase when the pedestrian entrance rate surpasses a critical value. However, raised crosswalk has lesser repercussions in CO2 emission and energy dissipation by eliciting traffic calm. This study enhances the comprehension of the impacts of mid-block crosswalks, depending on the manner in which they are deployed.
•Pedestrian–vehicle interactions at unsignalized mid-block zebra and raised crosswalks.•Characterization of the alterations promoted by raised and zebra crosswalks in terms of traffic flow, CO2 emissions, and kinetic energy dissipation.•Cellular automata model for analyzing pedestrian–vehicle interactions at mid-block crosswalks.
Traffic bottleneck is considered as one of the major causes of the disturbance in traffic flow. The understanding the dynamics between vehicles and bottlenecks is crucial for enhancing traffic flow ...and ensuring road safety. This research examines a two-lane traffic cellular automaton model to understand the effects of static (e.g., lane reductions) and dynamic (e.g., slow-moving vehicles) bottlenecks on traffic flow and road safety. We found that at low vehicle densities, slow vehicles gravitate towards the open lane, while faster vehicles switch lanes to overtake, returning to their original lane post-bottleneck. At high densities, traffic flow near static bottlenecks ceases, independent of bottleneck length. Safety analysis shows that extended static bottlenecks reduce rear-end collision risk due to fewer lane changes and increased vehicle stationarity. At maximum density, gridlock nullifies the chance of such collisions. Our findings provide actionable insights for traffic planning focused on bottleneck management to improve road safety.
Traffic emission is one of the most severe issues in our modern societies. A large part of emissions occurs in cities and especially at intersections due to the high interactions between vehicles. In ...this paper, we proposed a cellular automata model to investigate the different traffic emissions (CO2, PM, VOC, and NOx) and speeds at a two-lane signalized intersection. The model is designed to analyze the effects of signalization by isolating the parameters involved in vehicle-vehicle interactions (lane changing, speed, density, and traffic heterogeneity). It was found that the traffic emission increases (decreases) with the increasing of green lights duration (Tg) at low (high) values of vehicles injection rate (α). Moreover, by taking CO2 as the order parameter, the phase diagram shows that the system can be in four different phases (I, II, III, and IV) depending on α and Tg. The transition from phase II (I) to phase III (II) is second order, while the transition from phase II to phase IV is first order. To reduce the traffic emission and enhance the speed, two strategies were proposed. Simulation results show a maximum reduction of 13.6% in vehicles’ emissions and an increase of 9.5% in the mean speed when adopting self-organizing intersection (second strategy) at low and intermediate α. However, the first strategy enhances the mean speed up to 28.8% and reduces the traffic emissions by 3.6% at high α. Therefore, the combination of both strategies is recommended to promote the traffic efficiency in all traffic states. Finally, the model results illustrate that the system shows low traffic emission adopting symmetric lane-changing rules than asymmetric rules.
We present a systematic first-principles study of the structural, magnetic and optical properties of perovskite-structure EuTiO3. This compound exists in different structures: cubic, tetragonal and ...presents multiferroic properties. Comparing the formation energy between tetragonal and cubic structures, the system has a tendency to symmetry lowering structural deformations composed of rotations of the oxygen octahedral, especially the I4/mcm phase is the most stable structure. Our calculations of the high symmetry cubic structural prototype show an antiferromagnetic order type G. We discuss the dynamical stability of Pm-3m, P4mm and I4-mcm structures, and the influence of some parameters on the magnetic coupling and the electrical polarization.
Simulation Analysis of Traffic Management in Roundabout Systems Ez-zahar, Amal; Lakouari, Noureddine; Oubram, Outmane ...
2024 4th International Conference on Innovative Research in Applied Science, Engineering and Technology (IRASET),
2024-May-16
Conference Proceeding
Urban transport systems incorporating roundabouts often confront substantial difficulties, notably when vehicles entering the roundabout fail to maintain appropriate gaps with those in the ...circulating lane. To address such challenges, we suggest utilizing a cellular automata model, specifically the Nagel-Schreckenberg (NaSch) framework, for evaluating the flow dynamics within single-lane roundabouts governed by traffic sig-nal coordination. We explore two distinct operational conditions: firstly, a scenario wherein vehicles on the approach adhere to established precedence rules; secondly, a condition under which traffic signals dictate the flow. Simulation studies reveal that the first condition gives rise to a triphasic traffic flow pattern characterized by free flow, jammed, and maximum current phases. Notably, an increase in the number of vehicles ignoring the requisite safety gaps upon entering the roundabout diminishes the throughput of the circulating lane. Conversely, in the second condition, traffic light implementation modulates the flow phases within the roundabout system. Although the extent of the free-flow phase remains stable, the capacity during maximum current conditions intensifies. However, congestion phases are observed in all lanes of the roundabout. Furthermore, the variability in the timing of traffic signals, specifically the alteration between green and red phases, significantly affects the system's overall efficiency.
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