2,898 research outputs found
Random walk theory of jamming in a cellular automaton model for traffic flow
The jamming behavior of a single lane traffic model based on a cellular
automaton approach is studied. Our investigations concentrate on the so-called
VDR model which is a simple generalization of the well-known
Nagel-Schreckenberg model. In the VDR model one finds a separation between a
free flow phase and jammed vehicles. This phase separation allows to use random
walk like arguments to predict the resolving probabilities and lifetimes of jam
clusters or disturbances. These predictions are in good agreement with the
results of computer simulations and even become exact for a special case of the
model. Our findings allow a deeper insight into the dynamics of wide jams
occuring in the model.Comment: 16 pages, 7 figure
Reliability-based lifetime performance analysis of long-span bridges
2010 Fall.Includes bibliographical references.Long-span bridges generally serve as the significant hub in the transportation system for normal transportation and critical evacuation paths when any disaster happens. Thus, the safety and serviceability of long-span bridges are related to huge economic cost and safety of thousands of lives. The objective of this research is to establish a general framework to evaluate the lifetime performance of long-span bridges through taking account of more realistic load situations, such as traffic flow and wind environment. After some background information is introduced in Chapter 1, Chapter 2 covers the modeling of stochastic traffic flow for the bridge infrastructure system in a more realistic way by using the Cellular Automaton model. Based on the detailed information of individual vehicles of the stochastic traffic flow, the general framework to study Bridge/Traffic/Wind dynamic performance is developed in Chapter 3. Chapter 3 and Chapter 4 also report the results of the bridge's serviceability under normal and extreme loads events, respectively. In Chapter 5, the scenario-based fatigue model is further developed based on the dynamic framework developed in Chapter 3. Finally, the reliability-based analysis is conducted in Chapter 6 to study the fatigue damage caused by the coupling effects among bridge, traffic flow and wind throughout the bridge's service life
Cellular automata approach to three-phase traffic theory
The cellular automata (CA) approach to traffic modeling is extended to allow
for spatially homogeneous steady state solutions that cover a two dimensional
region in the flow-density plane. Hence these models fulfill a basic postulate
of a three-phase traffic theory proposed by Kerner. This is achieved by a
synchronization distance, within which a vehicle always tries to adjust its
speed to the one of the vehicle in front. In the CA models presented, the
modelling of the free and safe speeds, the slow-to-start rules as well as some
contributions to noise are based on the ideas of the Nagel-Schreckenberg type
modelling. It is shown that the proposed CA models can be very transparent and
still reproduce the two main types of congested patterns (the general pattern
and the synchronized flow pattern) as well as their dependence on the flows
near an on-ramp, in qualitative agreement with the recently developed continuum
version of the three-phase traffic theory [B. S. Kerner and S. L. Klenov. 2002.
J. Phys. A: Math. Gen. 35, L31]. These features are qualitatively different
than in previously considered CA traffic models. The probability of the
breakdown phenomenon (i.e., of the phase transition from free flow to
synchronized flow) as function of the flow rate to the on-ramp and of the flow
rate on the road upstream of the on-ramp is investigated. The capacity drops at
the on-ramp which occur due to the formation of different congested patterns
are calculated.Comment: 55 pages, 24 figure
Zero range model of traffic flow
A multi--cluster model of traffic flow is studied, in which the motion of
cars is described by a stochastic master equation. Assuming that the escape
rate from a cluster depends only on the cluster size, the dynamics of the model
is directly mapped to the mathematically well-studied zero-range process.
Knowledge of the asymptotic behaviour of the transition rates for large
clusters allows us to apply an established criterion for phase separation in
one-dimensional driven systems. The distribution over cluster sizes in our
zero-range model is given by a one--step master equation in one dimension. It
provides an approximate mean--field dynamics, which, however, leads to the
exact stationary state. Based on this equation, we have calculated the critical
density at which phase separation takes place. We have shown that within a
certain range of densities above the critical value a metastable homogeneous
state exists before coarsening sets in. Within this approach we have estimated
the critical cluster size and the mean nucleation time for a condensate in a
large system. The metastablity in the zero-range process is reflected in a
metastable branch of the fundamental flux--density diagram of traffic flow. Our
work thus provides a possible analytical description of traffic jam formation
as well as important insight into condensation in the zero-range process.Comment: 10 pages, 13 figures, small changes are made according to finally
accepted version for publication in Phys. Rev.
Parameter estimation for stochastic hybrid model applied to urban traffic flow estimation
This study proposes a novel data-based approach for estimating the parameters of a stochastic hybrid model describing the traffic flow in an urban traffic network with signalized intersections. The model represents the evolution of the traffic flow rate, measuring the number of vehicles passing a given location per time unit. This traffic flow rate is described using a mode-dependent first-order autoregressive (AR) stochastic process. The parameters of the AR process take different values depending on the mode of traffic operation – free flowing, congested or faulty – making this a hybrid stochastic process. Mode switching occurs according to a first-order Markov chain. This study proposes an expectation-maximization (EM) technique for estimating the transition matrix of this Markovian mode process and the parameters of the AR models for each mode. The technique is applied to actual traffic flow data from the city of Jakarta, Indonesia. The model thus obtained is validated by using the smoothed inference algorithms and an online particle filter. The authors also develop an EM parameter estimation that, in combination with a time-window shift technique, can be useful and practical for periodically updating the parameters of hybrid model leading to an adaptive traffic flow state estimator
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