Traffic Simulation
Encyclopedia
Traffic simulation or the simulation of transportation systems is the mathematical modeling
(US, modelling in UK British) of transportation systems (e.g., freeway junctions, arterial routes, roundabouts, downtown grid systems, etc.) through the application of computer software to better help plan, design and operate transportation systems. Simulation of transportation systems started over forty years ago, and is an important area of discipline in Traffic Engineering
and Transportation Planning
today. Various national and local transportation agencies, academic institutions and consulting firms use simulation to aid in their management of transportation networks.
Simulation in transportation is important because it can study models too complicated for analytical or numerical treatment, can be used for experimental studies, can study detailed relations that might be lost in analytical or numerical treatment and can produce attractive visual demomonstrations of present and future scenarios.
To understand simulation, it is important to understand the concept of system state, which is a set of variables that contains enough information to describe the evolution of the system over time. System state can be either discrete
or continuous
. Traffic simulation models are classified according to discrete and continuous time, state, and space.
One of the earliest discrete event simulation models is the Monte Carlo Simulation
, where it a series of random numbers are used to synthesise traffic conditions.
This was followed by the Cellular Automata
model that generates randomness from deterministic rules.
More recent methods use either discrete event simulation
or continuous-time simulation
. Discrete event simulation models are both stochastic
(with random components) and dynamic (time is a variable). Single server queues
for instance can be modeled very well using discrete event simulation, as servers are usually at a single location and so are discrete (e.g. traffic lights). Continuous time simulation, on the other hand, can solve the shortcoming of discrete event simulation where the model is required to have input, state and output trajectories within a time interval. The method requires the use of differential equations, specifically numerical integration methods. These equations can range from simple methods, such as Euler's method, to higher order Taylor's series methods, such as Heun's method
and Runge-Kutta.
A class of microscopic
continuous-time models, known as car-following models
, are also based on differential equations. Significant models include the Intelligent Driver Model
and Gipps' Model
. They model the behavior of each individual vehicle ("microscopic") in order to see its implications on the whole traffic system ("macroscopic"). Employing a numerical method with a car-following model (such as Gipps' with Heun's) can generate important information for traffic conditions, such as system delays and identification of bottlenecks.
can be used to develop a wider understanding of traffic demands over a broad geographic area, and predicting future traffic levels at different links (sections) in the network, incorporating different growth scenarios, with feedback loops to incorporate the effect of congestion on the distribution of trips.
use these models to evaluate what-if scenarios in the region, such as air quality to help develop land use
policies that lead to more sustainable travel
. On the other hand, modeling of transportation system operations and design focus on a smaller scale, such as a highway corridor and pinch-points. Lane types, signal timing and other traffic related questions are investigated to improve local system effectiveness and efficiency. While certain simulation models are specialized to model either operations or system planning, certain models have the capability to model both to some degree.
Whether it is for planning or for systems operations, simulations can be used for a variety of transportation modes
.
modeling, that deals with the logistics of container handling to improve system efficiency. Air transportation simulation primarily involves modeling of the airport terminal
operations (baggage handling, security checkpoint), and runway
operations.
network, since in reality modes are integrated and represent more complexities that each individual mode can overlook. Inter-modal network simulation can also better understand the impact of a certain network from a comprehensive perspective to more accurately represent its impact in order to realize important policy implications.
Simulation in transportation can also be integrated with urban environment simulation, where a large urban area is simulated which includes roadway networks, to better understand land use and other planning implications of the traffic network on the urban environment.
to obtain the desired accuracy. There will be a 'warm-up' period before the system reaches a steady state, and this period should be excluded from the results.
Microsimulation models usually produce two types of results: animated displays, and numerical output in text files. It is important to understand how the software has accumulated and summarized the numerical results to prevent incorrect interpretation. Animation can allow the analyst to quickly assess the performance, however it is limited to qualitative comparisons. The main indication of a problem that can be seen in an animation is the forming of persistent queues.
'Measures of Effectiveness' (MOEs) may be calculated or defined in a manner which is unique to each simulation program. MOEs are the system performance statistics that categorize the degree to which a particular alternative meets the project objectives. The following MOEs are most common when analyzing simulation models:
Other commonly reported metrics from traffic simulation tools include:
The HCM 2010 is currently a work in progress due to be released in late 2010 - early 2011. A major effort of this update to the HCM will be revised guidance on what types of output from traffic simulation software are most suitable for analysis in, and comparison to, the HCM for example vehicle trajectories and raw loop detector output.
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
(US, modelling in UK British) of transportation systems (e.g., freeway junctions, arterial routes, roundabouts, downtown grid systems, etc.) through the application of computer software to better help plan, design and operate transportation systems. Simulation of transportation systems started over forty years ago, and is an important area of discipline in Traffic Engineering
Traffic engineering
Traffic engineering can mean:* traffic engineering , a branch of civil engineering* teletraffic engineering, a field of statistical techniques used in telecommunications...
and Transportation Planning
Transportation planning
Transportation planning is a field involved with the evaluation, assessment, design and siting of transportation facilities .-Models and Sustainability :...
today. Various national and local transportation agencies, academic institutions and consulting firms use simulation to aid in their management of transportation networks.
Simulation in transportation is important because it can study models too complicated for analytical or numerical treatment, can be used for experimental studies, can study detailed relations that might be lost in analytical or numerical treatment and can produce attractive visual demomonstrations of present and future scenarios.
To understand simulation, it is important to understand the concept of system state, which is a set of variables that contains enough information to describe the evolution of the system over time. System state can be either discrete
Discrete time
Discrete time is the discontinuity of a function's time domain that results from sampling a variable at a finite interval. For example, consider a newspaper that reports the price of crude oil once every day at 6:00AM. The newspaper is described as sampling the cost at a frequency of once per 24...
or continuous
Continuous simulation
Continuous Simulation refers to a computer model of a physical system that continuously tracks system response over time according to a set of equations typically involving differential equations.-History:...
. Traffic simulation models are classified according to discrete and continuous time, state, and space.
Traffic models
Simulation methods in transportation can employ a selection of theories, including probability and statistics, differential equations and numerical methods.- Monte Carlo method
One of the earliest discrete event simulation models is the Monte Carlo Simulation
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...
, where it a series of random numbers are used to synthesise traffic conditions.
- Cellular Automata model
This was followed by the Cellular Automata
Cellular automaton
A cellular automaton is a discrete model studied in computability theory, mathematics, physics, complexity science, theoretical biology and microstructure modeling. It consists of a regular grid of cells, each in one of a finite number of states, such as "On" and "Off"...
model that generates randomness from deterministic rules.
- Discrete event and continuous-time simulation
More recent methods use either discrete event simulation
Discrete Event Simulation
In discrete-event simulation, the operation of a system is represented as a chronological sequence of events. Each event occurs at an instant in time and marks a change of state in the system...
or continuous-time simulation
Continuous simulation
Continuous Simulation refers to a computer model of a physical system that continuously tracks system response over time according to a set of equations typically involving differential equations.-History:...
. Discrete event simulation models are both stochastic
Stochastic
Stochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...
(with random components) and dynamic (time is a variable). Single server queues
Queueing model
In queueing theory, a queueing model is used to approximate a real queueing situation or system, so the queueing behaviour can be analysed mathematically...
for instance can be modeled very well using discrete event simulation, as servers are usually at a single location and so are discrete (e.g. traffic lights). Continuous time simulation, on the other hand, can solve the shortcoming of discrete event simulation where the model is required to have input, state and output trajectories within a time interval. The method requires the use of differential equations, specifically numerical integration methods. These equations can range from simple methods, such as Euler's method, to higher order Taylor's series methods, such as Heun's method
Heun's method
In mathematics and computational science, Heun's method may refer to the improved or modified Euler's method , or a similar two-stage Runge–Kutta method. It is named after Karl L. W. M. Heun and is a numerical procedure for solving ordinary differential equations with a given initial value...
and Runge-Kutta.
- Car-following models
A class of microscopic
Microscopic traffic flow model
Microscopic traffic flow models are a class of scientific models of vehicular traffic dynamics.In contrast to macroscopic models, microscopic traffic flow models simulate single vehicle-driver units, thus the dynamic variables of the models represent microscopic properties like the position and...
continuous-time models, known as car-following models
Microscopic traffic flow model
Microscopic traffic flow models are a class of scientific models of vehicular traffic dynamics.In contrast to macroscopic models, microscopic traffic flow models simulate single vehicle-driver units, thus the dynamic variables of the models represent microscopic properties like the position and...
, are also based on differential equations. Significant models include the Intelligent Driver Model
Intelligent Driver Model
In traffic flow modeling, the intelligent driver model is a time-continuous car-following model for the simulation of freeway and urban traffic...
and Gipps' Model
Gipps' Model
Gipps’ Model is a car-following model named after Peter G. Gipps who developed it in the late-1970s under S.R.C. grants at the at the University of Newcastle-Upon-Tyne and the Transport Studies Group at the University College London. It is based directly on driver behavior and expectancy for...
. They model the behavior of each individual vehicle ("microscopic") in order to see its implications on the whole traffic system ("macroscopic"). Employing a numerical method with a car-following model (such as Gipps' with Heun's) can generate important information for traffic conditions, such as system delays and identification of bottlenecks.
Systems Planning
The methods noted above are generally used to model the behavior of an existing system, and are often focused around specific areas of interest under a range of conditions (such as a change in layout, lane closures, and different levels of traffic flow). Transport planning and forecastingTransportation forecasting
Transportation forecasting is the process of estimating the number of vehicles or people that will use a specific transportation facility in the future. For instance, a forecast may estimate the number of vehicles on a planned road or bridge, the ridership on a railway line, the number of...
can be used to develop a wider understanding of traffic demands over a broad geographic area, and predicting future traffic levels at different links (sections) in the network, incorporating different growth scenarios, with feedback loops to incorporate the effect of congestion on the distribution of trips.
Applications in Transportation Engineering
Traffic simulation models are useful from a microscopic, macroscopic and sometimes mesoscopic perspectives. Simulation can be applied to both transportation planning and to transportation design and operations. In transportation planning the simulation models evaluates the impacts of regional urban development patterns on the performance of the transportation infrastructure. Regional planning organizationsMetropolitan planning organization
A metropolitan planning organization is a federally-mandated and federally-funded transportation policy-making organization in the United States that is made up of representatives from local government and governmental transportation authorities...
use these models to evaluate what-if scenarios in the region, such as air quality to help develop land use
Land use
Land use is the human use of land. Land use involves the management and modification of natural environment or wilderness into built environment such as fields, pastures, and settlements. It has also been defined as "the arrangements, activities and inputs people undertake in a certain land cover...
policies that lead to more sustainable travel
Sustainable transport
Sustainable transport refers to any means of transport with low impact on the environment, and includes walking and cycling, transit oriented development, green vehicles, CarSharing, and building or protecting urban transport systems that are fuel-efficient, space-saving and promote healthy...
. On the other hand, modeling of transportation system operations and design focus on a smaller scale, such as a highway corridor and pinch-points. Lane types, signal timing and other traffic related questions are investigated to improve local system effectiveness and efficiency. While certain simulation models are specialized to model either operations or system planning, certain models have the capability to model both to some degree.
Whether it is for planning or for systems operations, simulations can be used for a variety of transportation modes
Mode of transport
Mode of transport is a term used to distinguish substantially different ways to perform transport. The most dominant modes of transport are aviation, land transport, which includes rail, road and off-road transport, and ship transport...
.
Roadway/Ground Transportation
Ground transportation for both passenger and goods movement is perhaps the area where simulation is most widely used. Simulation can be carried out at a corridor level, or at a more complex roadway grid network level to analyze planning, design and operations such as delay, pollution, and congestion. Ground transportation models can include all modes of roadway travel, including vehicles, trucks, buses, bicycles and pedestrians. In traditional road traffic models, aggregate representation of traffic is typically used where all vehicles of a particular group obey the same rules of behavior; in micro-simulation, driver behavior and network performance are included so that complete traffic problems (e.g. Intelligent transportation system, shockwaves) can be examined.Rail Transportation
Rail is an important mode of travel for both freight and passengers. Modeling railways for freight movement is important to determine the operational efficiency and rationalize planning decisions. Freight simulation can include aspects such as dedicated truck lanes, commodity flow, corridor and system capacity, traffic assignment/network flow, and freight plans that involve travel demand forecasting.Maritime and Air Transportation
Maritime and air transportation presents two areas that are important for the economy. Maritime simulation primarily includes container terminalContainer terminal
A container terminal is a facility where cargo containers are transshipped between different transport vehicles, for onward transportation. The transshipment may be between container ships and land vehicles, for example trains or trucks, in which case the terminal is described as a maritime...
modeling, that deals with the logistics of container handling to improve system efficiency. Air transportation simulation primarily involves modeling of the airport terminal
Airport terminal
An airport terminal is a building at an airport where passengers transfer between ground transportation and the facilities that allow them to board and disembark from aircraft....
operations (baggage handling, security checkpoint), and runway
Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...
operations.
Other
In addition to simulating individual modes, it is often more important to simulate a multi-modalIntermodal transport
Intermodal transport involves the use of more than one mode of transport for a journey. See:*Intermodal passenger transport*Intermodal freight transport...
network, since in reality modes are integrated and represent more complexities that each individual mode can overlook. Inter-modal network simulation can also better understand the impact of a certain network from a comprehensive perspective to more accurately represent its impact in order to realize important policy implications.
Simulation in transportation can also be integrated with urban environment simulation, where a large urban area is simulated which includes roadway networks, to better understand land use and other planning implications of the traffic network on the urban environment.
Software Programs
Simulation software is getting better in a variety of different ways. With new advancements in mathematics, engineering and computing, simulation software programs are increasingly becoming faster, more powerful, more detail oriented and more realistic. Below is a list of well known simulation software for transportation.Macroscopic
- Emme/2
- OREMS
- TransCAD
- http://www.caliper.comTransModelerTransModelerTransModeler is the name of a traffic simulation product for doing wide-area traffic planning, traffic management, and emergency evacuation studies...
] - DYNEV
- OmniTRANS
- VISUM
Mesoscopic
- Cube
- DYNASMART
- TRANSIMSTransimsTRANSIMS , is an integrated set of tools developed to conduct regional transportation system analyses...
- http://www.caliper.comTransModelerTransModelerTransModeler is the name of a traffic simulation product for doing wide-area traffic planning, traffic management, and emergency evacuation studies...
]
Microscopic
- http://www.tss-bcn.com/aimsun.htmlAimsunAimsunAimsun is an integrated transport modelling software, developed and marketed by TSS - Transport Simulation Systems based in Barcelona, Spain.Aimsun software is used by government agencies, municipalities, universities and consultants worldwide for traffic engineering, traffic simulation,...
] - ARCADYArcadyARCADY is the acronym for . This software, produced by the Transport Research Laboratory , provides information on traffic flow including modelling capacity queues and delays at roundabouts...
- CityTrafficSimulator
- CORSIM
- DRACULA
- DYNASIM
- http://www.matsim.orgMATSimMATSimMATSim is an open source software development project developing agent-based software modules intended for use with transportation planning models....
] - http://www.paramics-online.comQuadstone ParamicsQuadstone ParamicsParamics is traffic microsimulation software developed by Quadstone Paramics. Quadstone Paramics also develops pedestrian microsimulation software called the Urban Analytics Framework.-Background:...
] - SimTraffic
- http://www.caliper.comTransModelerTransModelerTransModeler is the name of a traffic simulation product for doing wide-area traffic planning, traffic management, and emergency evacuation studies...
] - TRITONE_(simulazione)
- UAF
- http://www.ptvamerica.comVISSIMVISSIMVISSIM is a microscopic multi-modal traffic flow simulation software. It is developed by PTV Planung Transport Verkehr AG in Karlsruhe, Germany. The name is derived from “Verkehr In Städten - SIMulationsmodell”...
]
Microsimulation
Microsimulation models track individual vehicle movements on a second or subsecond basis. Microsimulation relies on random numbers to generate vehicles, select routing decisions, and determine behavior. Because of this variation, it is necessary to run the model several times with different random number seedsRandom seed
A random seed is a number used to initialize a pseudorandom number generator.The choice of a good random seed is crucial in the field of computer security...
to obtain the desired accuracy. There will be a 'warm-up' period before the system reaches a steady state, and this period should be excluded from the results.
Microsimulation models usually produce two types of results: animated displays, and numerical output in text files. It is important to understand how the software has accumulated and summarized the numerical results to prevent incorrect interpretation. Animation can allow the analyst to quickly assess the performance, however it is limited to qualitative comparisons. The main indication of a problem that can be seen in an animation is the forming of persistent queues.
'Measures of Effectiveness' (MOEs) may be calculated or defined in a manner which is unique to each simulation program. MOEs are the system performance statistics that categorize the degree to which a particular alternative meets the project objectives. The following MOEs are most common when analyzing simulation models:
- 'VMT' (Vehicle Miles Traveled) is computed as a combination of the number of vehicles in the system and their distance they traveled.
- 'VHT' (Vehicle Hours of Travel) is computed as the product of the link volume and the link travel time, summed over all links.
- 'Mean system speed' is equal to VMT/VHT.
- 'Total system delay' is one of the most effective ways to evaluate different congestion relieving alternatives and it is usually the MOE that the travelling public notices. Delay can be calculated several ways. Some consider it to be only that delay which is above free flow conditions. Others include the baseline delay which occurs as a result of traffic control devices. Some even include acceleration and deceleration delay, while others include only stopped delay.
Other commonly reported metrics from traffic simulation tools include:
- Link road section speeds, flow, density, travel time, delay, stop time
- Intersection turning volumes, delay,
- Journey times
- loop detector records for speed, occupancy, headway, gap
- vehicle trajectories and speed vs. distance plots
Comparing simulation results with the US Highway Capacity Manual
The output of a microsimulation model is different from that of the US Federal Highway Capacity Manual (HCM). For example, most HCM procedures assume that the operation of one intersection will not be affected by the conditions of an adjacent roadway (with the exception of HCS 2000 Freeways). 'Rubbernecking' and long queues from one location interfering with another location would contradict this assumption.The HCM 2010 is currently a work in progress due to be released in late 2010 - early 2011. A major effort of this update to the HCM will be revised guidance on what types of output from traffic simulation software are most suitable for analysis in, and comparison to, the HCM for example vehicle trajectories and raw loop detector output.