35 research outputs found
A stochastic model of randomly accelerated walkers for human mobility
The recent availability of large databases allows to study macroscopic
properties of many complex systems. However, inferring a model from a fit of
empirical data without any knowledge of the dynamics might lead to erroneous
interpretations [6]. We illustrate this in the case of human mobility [1-3] and
foraging human patterns [4] where empirical long-tailed distributions of jump
sizes have been associated to scale-free super-diffusive random walks called
L\'evy flights [5]. Here, we introduce a new class of accelerated random walks
where the velocity changes due to acceleration kicks at random times, which
combined with a peaked distribution of travel times [7], displays a jump length
distribution that could easily be misinterpreted as a truncated power law, but
that is not governed by large fluctuations. This stochastic model allows us to
explain empirical observations about the movements of 780,000 private vehicles
in Italy, and more generally, to get a deeper quantitative understanding of
human mobility.Comment: 10 pages, 6 figures + Supplementary informatio
Entropic measures of individual mobility patterns
Understanding human mobility from a microscopic point of view may represent a
fundamental breakthrough for the development of a statistical physics for
cognitive systems and it can shed light on the applicability of macroscopic
statistical laws for social systems. Even if the complexity of individual
behaviors prevents a true microscopic approach, the introduction of mesoscopic
models allows the study of the dynamical properties for the non-stationary
states of the considered system. We propose to compute various entropy measures
of the individual mobility patterns obtained from GPS data that record the
movements of private vehicles in the Florence district, in order to point out
new features of human mobility related to the use of time and space and to
define the dynamical properties of a stochastic model that could generate
similar patterns. Moreover, we can relate the predictability properties of
human mobility to the distribution of time passed between two successive trips.
Our analysis suggests the existence of a hierarchical structure in the mobility
patterns which divides the performed activities into three different
categories, according to the time cost, with different information contents. We
show that a Markov process defined by using the individual mobility network is
not able to reproduce this hierarchy, which seems the consequence of different
strategies in the activity choice. Our results could contribute to the
development of governance policies for a sustainable mobility in modern cities
Statistical Laws in Urban Mobility from microscopic GPS data in the area of Florence
The application of Statistical Physics to social systems is mainly related to
the search for macroscopic laws, that can be derived from experimental data
averaged in time or space,assuming the system in a steady state. One of the
major goals would be to find a connection between the statistical laws to the
microscopic properties: for example to understand the nature of the microscopic
interactions or to point out the existence of interaction networks. The
probability theory suggests the existence of few classes of stationary
distributions in the thermodynamics limit, so that the question is if a
statistical physics approach could be able to enroll the complex nature of the
social systems. We have analyzed a large GPS data base for single vehicle
mobility in the Florence urban area, obtaining statistical laws for path
lengths, for activity downtimes and for activity degrees. We show also that
simple generic assumptions on the microscopic behavior could explain the
existence of stationary macroscopic laws, with an universal function describing
the distribution. Our conclusion is that understanding the system complexity
requires dynamical data-base for the microscopic evolution, that allow to solve
both small space and time scales in order to study the transients.Comment: 17 pages, 14 figures .jpg, use imsart.cl
Multiple equilibria and their stability in a barotropic and baroclinic atmosphere
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982.Microfiche copy available in Archives and ScienceIncludes bibliographies.by Sandro Rambaldi.Ph.D
Understanding the variability of daily travel-time expenditures using GPS trajectory data
12+6 Pages, 6+2 Figures, 1+1 TablesTransportation planning is strongly influenced by the assumption that every individual has for his daily mobility a constant daily budget of ~1 hour. However, recent experimental results are proving this assumption as wrong. Here, we study the differences in daily travel-time expenditures among 24 Italian cities, extracted from a large set of GPS data on vehicles mobility. To understand these variations at the level of individual behaviour, we introduce a trip duration model that allows for a description of the distribution of travel-time expenditures in a given city using two parameters. The first parameter reflects the accessibility of desired destinations, whereas the second one can be associated to a travel-time budget and represents physiological limits due to stress and fatigue. Within the same city, we observe variations in the distributions according to home position, number of mobility days and a driver's average number of daily trips. These results can be interpreted by a stochastic time-consumption model, where the generalised cost of travel times is given by a logarithmic-like function, in agreement with the Weber-Fechner law. Our experimental results show a significant variability in the travel-time budgets in different cities and for different categories of drivers within the same city. This explicitly clashes with the idea of the existence of a constant travel-time budget and opens new perspectives for the modeling and governance of urban mobility
Frequency map analysis of resonances in a nonlinear lattice with space charge
Abstract In storage rings for heavy ion fusion beam losses must be minimized. During bunch compression high space charge is reached and the reciprocal effects between the collective modes of the beam and the single particle lattice nonlinearities must be considered to understand the problem of resonance crossing and halo formation. We show that the frequency map analysis of particle in core models gives an adequate description of the resonance network and of the chaotic regions where the halo particles can diffuse
3D solutions of the Poisson-Vlasov equations for a charged plasma and particle-core model in a line of FODO cells
We consider a charged plasma of positive ions in a periodic focusing channel of quadrupolar magnets in the presence of RF cavities. The ions are bunched into charged triaxial ellipsoids and their description requires the solution of a fully 3D Poisson-Vlasov equation. We also analyze the trajectories of test particles in the exterior of the ion bunches in order to estimate their diffusion rate. This rate is relevant for a high intensity linac (TRASCO project). A numerical PIC scheme to integrate the Poisson-Vlasov equations in a periodic focusing system in 2 and 3 space dimensions is presented. The scheme consists of a single particle symplectic integrator and a Poisson solver based on FFT plus tri-diagonal matrix inversion. In the 2D version arbitrary boundary conditions can be chosen. Since no analytical self-consistent 3D solution is known, we chose an initial Neuffer-KV distribution in phase space, whose electric field is close to the one generated by a uniformly filled ellipsoid. For a matched (periodic) beam the orbits of test particles moving in the field of an ellipsoidal bunch, whose semi-axis satisfy the envelope equations, is similar to the orbits generated by the self-consistent charge distribition obtained from the PIC simulation, even though it relaxes to a Fermi-Dirac-like distribution. After a transient the RMS radii and emittances have small amplitude oscillations. The PIC simulations for a mismatched (quasiperiodic) beam are no longer comparable with the ellipsoidal bunch model even though the qualitative behavior is the same, namely a stronger diffusion due to the increase of resonances
Transverse self-consistent modeling of a 3D bunch in SIS100 with micromap
We present the upgrade of the MICROMAP beam dynamics simulation library to include a 2 1/2 D space charge modeling of a 3D bunch using local slices in z. We discuss the parallelization technique, the performances, several tests and comparison with existing well-established analytical/numerical results in order to validate the code. An application to the SIS 100 synchrotron of the FAIR project at GSI is outlined
Towards a Statistical Physics of Human Mobility
In this paper, we extend some ideas of statistical physics to describe the
properties of human mobility. From a physical point of view, we consider the
statistical empirical laws of private cars mobility, taking advantage of a GPS
database which contains a sampling of the individual trajectories of 2% of the
whole vehicle population in an Italian region. Our aim is to discover possible
"universal laws" that can be related to the dynamical cognitive features of
individuals. Analyzing the empirical trip length distribution we study if the
travel time can be used as universal cost function in a mesoscopic model of
mobility. We discuss the implications of the elapsed times distribution between
successive trips that shows an underlying Benford's law, and we study the rank
distribution of the average visitation frequency to understand how people
organize their daily agenda. We also propose simple stochastic models to
suggest possible explanations of the empirical observations and we compare our
results with analogous results on statistical properties of human mobility
presented in the literature
Unraveling pedestrian mobility on a road network using ICTs data during great tourist events
Tourist flows in historical cities are continuously growing in a globalized world and adequate governance processes, politics and tools are necessary in order to reduce impacts on the urban livability and to guarantee the preservation of cultural heritage. The ICTs offer the possibility of collecting large amount of data that can point out and quantify some statistical and dynamic properties of human mobility emerging from the individual behavior and referring to a whole road network. In this paper we analyze a new dataset that has been collected by the Italian mobile phone company TIM, which contains the GPS positions of a relevant sample of mobile devices when they actively connected to the cell phone network. Our aim is to propose innovative tools allowing to study properties of pedestrian mobility on the whole road network. Venice is a paradigmatic example for the impact of tourist flows on the resident life quality and on the preservation of cultural heritage. The GPS data provide anonymized georeferenced information on the displacements of the devices. After a filtering procedure, we develop specific algorithms able to reconstruct the daily mobility paths on the whole Venice road network. The statistical analysis of the mobility paths suggests the existence of a travel time budget for the mobility and points out the role of the rest times in the empirical relation between the mobility time and the corresponding path length. We succeed to highlight two connected mobility subnetworks extracted from the whole road network, that are able to explain the majority of the observed mobility. Our approach shows the existence of characteristic mobility paths in Venice for the tourists and for the residents. Moreover the data analysis highlights the different mobility features of the considered case studies and it allows to detect the mobility paths associated to different points of interest. Finally we have disaggregated the Italian and foreigner categories to study their different mobility behaviors