Transportation forecasting
Encyclopedia
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 passengers visiting an airport
, or the number of ships calling on a seaport. Traffic forecasting begins with the collection of data
on current traffic. This traffic data is combined with other known data, such as population, employment, trip rates, travel costs, etc., to develop a traffic demand
model
for the current situation. Feeding it with predicted data for population, employment, etc. results in estimates of future traffic, typically estimated for each segment of the transportation infrastructure in question, e.g., for each roadway segment or railway station.
Traffic forecasts are used for several key purposes in transportation policy
, planning
, and engineering: to calculate the capacity of infrastructure, e.g., how many lanes a bridge should have; to estimate the financial and social viability
of projects, e.g., using cost-benefit analysis
and social impact assessment
; and to calculate environmental impacts
, e.g., air pollution
and noise
.
Area Transportation Study and Chicago Area Transportation Study (CATS).
Land use forecasting
starts the process. Typically, forecasts are made for the region as a whole, e.g., of population growth. Such forecasts provide control totals for the local land use analysis. Typically, the region is divided into zones and by trend or regression analysis
, the population and employment are determined for each.
The four steps of the classical urban transportation planning system model are:
After the classical model, there is an evaluation according to an agreed set of decision criteria and parameters. A typical criterion is cost-benefit analysis
. Such analysis might be applied after the network assignment model identifies needed capacity: is such capacity worthwhile? In addition to identifying the forecasting and decision steps as additional steps in the process, it is important to note that forecasting and decision-making permeate each step in the UTP process. Planning deals with the future, and it is forecasting dependent.
(e.g. work, leisure, shopping, ...) are conducted.
Partial and fully operational activity-based micro simulation systems include:
The major premise behind activity-based models is that travel demand is derived from activities that people need or wish to perform, with travel decisions forming part of the scheduling decisions. Travel is then seen as just one of the attributes of a system. The travel model is therefore set within the context of an agenda, as a component of an activity scheduling decision.
Activity-based models offer other possibilities than four-step models, e.g. to model environmental issues such as emissions and exposure to air pollution. Although their obvious advantages for environmental purposes were recognized by Shiftan almost a decade ago, applications to exposure models remain scarce. Activity-based models have recently been used to predict emissions and air quality.
They can also provide a better total estimate of exposure while also enabling the disaggregation of individual exposure over activities.
They can therefore be used to reduce exposure misclassification and establish relationships between health impacts and air quality more precisely.
Policy makers can use activity-based models to devise strategies that reduce exposure by changing time activity patterns or that target specific groups in the population.
and land use data are obtained, along with home interview surveys and journey surveys. Home interview surveys, land use data, and special trip attraction surveys provide the information on which the UTP analysis tools are exercised.
Data collection, management, and processing; model estimation; and use of models to yield plans are much used techniques in the UTP process. In the early days, in the USA, census data was augmented that with data collection methods that had been developed by the Bureau of Public Roads (a predecessor of the Federal Highway Administration
): traffic counting procedures, cordon "where are you coming from and where are you going" counts, and home interview techniques. Protocols for coding networks and the notion of analysis or traffic zones emerged at the CATS.
Model estimation used existing techniques, and plans were developed using whatever models had been developed in a study. The main difference between now and then is the development of some analytic resources specific to transportation planning, in addition to the BPR data acquisition techniques used in the early days.
project at Los Alamos National Laboratory
, developed by physicists. While the use of supercomputers and the detailed simulations may be an improvement on practice, they have yet to be shown to be better (more accurate) than conventional models. A commercial version was spun off to IBM, and an open source
version is also being actively maintained as TRANSIMS Open-Source.
One of the major oversights in the use of transportation models in practice is the absence of any feedback from transportation models on land use. Highways and transit investments not only respond to land use, they shape it as well.
s that typically form the basis for decisions on whether to build new transportation infrastructure or not.
To date there has been little research into this area. However, a peer-reviewed study of a large number of traffic forecasts found that a significant number of forecasts are inaccurate. In particular:
Measured over decades, a scheme can have such a large turnover that even a small percentage change in the projected traffic can indicate a significant positive or negative economic effect. For road schemes, the study noted that changes in land use and difficulties in estimating journeys are often blamed for these errors. Sensitivity analysis is normally carried out to understand the performance of the scheme if the parameters change; but subsequent official policy decisions can also have a major effect, such as changes to the area plan, discount rates and values of time. For rail schemes, inaccuracies are often blamed on uncertain measurement of passenger journeys and deliberately slanted forecasts by over-optimistic promoters.
Reference class forecasting
was developed to identify inaccuracy in forecasting with its first application being the Edinburgh tram system in 2004.
Infrastructure
Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...
in the future. For instance, a forecast may estimate the number of vehicles on a planned road or bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
, the ridership on a railway line, the number of passengers visiting an airport
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...
, or the number of ships calling on a seaport. Traffic forecasting begins with the collection of data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
on current traffic. This traffic data is combined with other known data, such as population, employment, trip rates, travel costs, etc., to develop a traffic demand
Demand
- Economics :*Demand , the desire to own something and the ability to pay for it*Demand curve, a graphic representation of a demand schedule*Demand deposit, the money in checking accounts...
model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
for the current situation. Feeding it with predicted data for population, employment, etc. results in estimates of future traffic, typically estimated for each segment of the transportation infrastructure in question, e.g., for each roadway segment or railway station.
Traffic forecasts are used for several key purposes in transportation policy
Policy
A policy is typically described as a principle or rule to guide decisions and achieve rational outcome. The term is not normally used to denote what is actually done, this is normally referred to as either procedure or protocol...
, planning
Transportation planning
Transportation planning is a field involved with the evaluation, assessment, design and siting of transportation facilities .-Models and Sustainability :...
, and engineering: to calculate the capacity of infrastructure, e.g., how many lanes a bridge should have; to estimate the financial and social viability
Viability
Viable or viability is the ability of a thing to maintain itself or recover its potentialities.Viable or viability may also refer to:...
of projects, e.g., using cost-benefit analysis
Cost-benefit analysis
Cost–benefit analysis , sometimes called benefit–cost analysis , is a systematic process for calculating and comparing benefits and costs of a project for two purposes: to determine if it is a sound investment , to see how it compares with alternate projects...
and social impact assessment
Social impact assessment
Social impact assessment is a methodology to review the social effects of infrastructure projects and other development interventions.-Definition:...
; and to calculate environmental impacts
Environmental degradation
Environmental degradation is the deterioration of the environment through depletion of resources such as air, water and soil; the destruction of ecosystems and the extinction of wildlife...
, e.g., air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
and noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
.
Four-step models
Within the rational planning framework, transportation forecasts have traditionally followed the sequential four-step model or urban transportation planning (UTP) procedure, first implemented on mainframe computers in the 1950s at the DetroitDetroit, Michigan
Detroit is the major city among the primary cultural, financial, and transportation centers in the Metro Detroit area, a region of 5.2 million people. As the seat of Wayne County, the city of Detroit is the largest city in the U.S. state of Michigan and serves as a major port on the Detroit River...
Area Transportation Study and Chicago Area Transportation Study (CATS).
Land use forecasting
Land use forecasting
Land-use forecasting undertakes to project the distribution and intensity of trip generating activities in the urban area. In practice, land-use models are demand-driven, using as inputs the aggregate information on growth produced by an aggregate economic forecasting activity...
starts the process. Typically, forecasts are made for the region as a whole, e.g., of population growth. Such forecasts provide control totals for the local land use analysis. Typically, the region is divided into zones and by trend or regression analysis
Regression analysis
In statistics, regression analysis includes many techniques for modeling and analyzing several variables, when the focus is on the relationship between a dependent variable and one or more independent variables...
, the population and employment are determined for each.
The four steps of the classical urban transportation planning system model are:
- Trip generationTrip generationTrip generation is the first step in the conventional four-step transportation forecasting process , widely used for forecasting travel demands...
determines the frequency of origins or destinations of trips in each zone by trip purpose, as a function of land uses and household demographics, and other socio-economic factors. - Trip distributionTrip distributionTrip distribution , is the second component in the traditional four-step transportation forecasting model...
matches origins with destinations, often using a gravity modelGravity modelGravity models are used in various social sciences to predict and describe certain behaviors that mimic gravitational interaction as described in Isaac Newton's law of gravity...
function, equivalent to an entropy maximizing modelPrinciple of maximum entropyIn Bayesian probability, the principle of maximum entropy is a postulate which states that, subject to known constraints , the probability distribution which best represents the current state of knowledge is the one with largest entropy.Let some testable information about a probability distribution...
. Older models include the fratar model. - Mode choiceMode choiceMode choice analysis is the third step in the conventional four-step transportation forecasting model, following trip generation and trip distribution but before route assignment. Trip distribution's zonal interchange analysis yields a set of origin destination tables which tells where the trips...
computes the proportion of trips between each origin and destination that use a particular transportation mode. (This modal model may be of the logitLogitThe logit function is the inverse of the sigmoidal "logistic" function used in mathematics, especially in statistics.Log-odds and logit are synonyms.-Definition:The logit of a number p between 0 and 1 is given by the formula:...
form, developed by Nobel Prize winner Daniel McFaddenDaniel McFaddenDaniel Little McFadden is an econometrician who shared the 2000 Nobel Memorial Prize in Economic Sciences with James Heckman ; McFadden's share of the prize was "for his development of theory and methods for analyzing discrete choice". He was the E. Morris Cox Professor of Economics at the...
.) - Route assignmentRoute assignmentRoute assignment, route choice, or traffic assignment concerns the selection of routes between origins and destinations in transportation networks. It is the fourth step in the conventional transportation forecasting model, following trip generation, trip distribution, and mode choice...
allocates trips between an origin and destination by a particular mode to a route. Often (for highway route assignment) WardropJohn Glen WardropJohn Glen Wardrop was an English transport analyst who developed Wardrop's first and second principles of equilibrium.The concepts are related to the idea of Nash equilibrium in game theory developed separately...
's principle of user equilibrium is applied (equivalent to a Nash equilibriumNash equilibriumIn game theory, Nash equilibrium is a solution concept of a game involving two or more players, in which each player is assumed to know the equilibrium strategies of the other players, and no player has anything to gain by changing only his own strategy unilaterally...
), wherein each driver (or group) chooses the shortest (travel time) path, subject to every other driver doing the same. The difficulty is that travel times are a function of demand, while demand is a function of travel time, the so-called bi-level problem. Another approach is to use the Stackelberg competitionStackelberg competitionThe Stackelberg leadership model is a strategic game in economics in which the leader firm moves first and then the follower firms move sequentially...
model, where users ("followers") respond to the actions of a "leader", in this case for example a traffic manager. This leader anticipates on the response of the followers.
After the classical model, there is an evaluation according to an agreed set of decision criteria and parameters. A typical criterion is cost-benefit analysis
Cost-benefit analysis
Cost–benefit analysis , sometimes called benefit–cost analysis , is a systematic process for calculating and comparing benefits and costs of a project for two purposes: to determine if it is a sound investment , to see how it compares with alternate projects...
. Such analysis might be applied after the network assignment model identifies needed capacity: is such capacity worthwhile? In addition to identifying the forecasting and decision steps as additional steps in the process, it is important to note that forecasting and decision-making permeate each step in the UTP process. Planning deals with the future, and it is forecasting dependent.
Activity-based models
Activity-based models are another class of models that predict for individuals where and when specific activities(e.g. work, leisure, shopping, ...) are conducted.
Partial and fully operational activity-based micro simulation systems include:
- MIDAS (Micro-analytic Integrated Demographic Accounting System)
- CEMDAP
- PCATS (Prism Constrained Activity-Travel Simulator)
- SIMAP
- ALBATROSS ("A Learning-Based Transportation Oriented Simulation System") model.
- FAMOS (Florida’s Activity Mobility Simulator)
- TASHA (Travel Activity Scheduler for Household agents)
- the Best Practice Models of the New York Metropolitan Transportation Council, Columbus and San Francisco County
The major premise behind activity-based models is that travel demand is derived from activities that people need or wish to perform, with travel decisions forming part of the scheduling decisions. Travel is then seen as just one of the attributes of a system. The travel model is therefore set within the context of an agenda, as a component of an activity scheduling decision.
Activity-based models offer other possibilities than four-step models, e.g. to model environmental issues such as emissions and exposure to air pollution. Although their obvious advantages for environmental purposes were recognized by Shiftan almost a decade ago, applications to exposure models remain scarce. Activity-based models have recently been used to predict emissions and air quality.
They can also provide a better total estimate of exposure while also enabling the disaggregation of individual exposure over activities.
They can therefore be used to reduce exposure misclassification and establish relationships between health impacts and air quality more precisely.
Policy makers can use activity-based models to devise strategies that reduce exposure by changing time activity patterns or that target specific groups in the population.
Precursor steps
Although not identified as steps in the UTP process, a lot of data gathering is involved in the UTP analysis process. CensusCensus
A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common...
and land use data are obtained, along with home interview surveys and journey surveys. Home interview surveys, land use data, and special trip attraction surveys provide the information on which the UTP analysis tools are exercised.
Data collection, management, and processing; model estimation; and use of models to yield plans are much used techniques in the UTP process. In the early days, in the USA, census data was augmented that with data collection methods that had been developed by the Bureau of Public Roads (a predecessor of the Federal Highway Administration
Federal Highway Administration
The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two "programs," the Federal-aid Highway Program and the Federal Lands Highway Program...
): traffic counting procedures, cordon "where are you coming from and where are you going" counts, and home interview techniques. Protocols for coding networks and the notion of analysis or traffic zones emerged at the CATS.
Model estimation used existing techniques, and plans were developed using whatever models had been developed in a study. The main difference between now and then is the development of some analytic resources specific to transportation planning, in addition to the BPR data acquisition techniques used in the early days.
Critique
The sequential and aggregate nature of transportation forecasting has come under much criticism. While improvements have been made, in particular giving an activity-base to travel demand, much remains to be done. In the 1990s, most federal investment in model research went to the TransimsTransims
TRANSIMS , is an integrated set of tools developed to conduct regional transportation system analyses...
project at Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
, developed by physicists. While the use of supercomputers and the detailed simulations may be an improvement on practice, they have yet to be shown to be better (more accurate) than conventional models. A commercial version was spun off to IBM, and an open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
version is also being actively maintained as TRANSIMS Open-Source.
One of the major oversights in the use of transportation models in practice is the absence of any feedback from transportation models on land use. Highways and transit investments not only respond to land use, they shape it as well.
Inaccuracy
Accurate traffic forecasts are critical to arriving at the right capacity for transportation infrastructure, that is, for building infrastructure that is neither too large or too small to meet the demand over a given period. Accurate traffic forecasts are also critical to obtaining valid results from the cost-benefit analyses, environmental impact assessments, and social impact assessmentSocial impact assessment
Social impact assessment is a methodology to review the social effects of infrastructure projects and other development interventions.-Definition:...
s that typically form the basis for decisions on whether to build new transportation infrastructure or not.
To date there has been little research into this area. However, a peer-reviewed study of a large number of traffic forecasts found that a significant number of forecasts are inaccurate. In particular:
- for nine out of ten railway projects the study found that passenger forecasts were overestimated, with an average overestimate of 106%,
- for half of all road projects, including bridges and tunnels, the study found that the difference between actual and forecast traffic was more than 20%, while for 25% of road projects the difference was more than 40%.
Measured over decades, a scheme can have such a large turnover that even a small percentage change in the projected traffic can indicate a significant positive or negative economic effect. For road schemes, the study noted that changes in land use and difficulties in estimating journeys are often blamed for these errors. Sensitivity analysis is normally carried out to understand the performance of the scheme if the parameters change; but subsequent official policy decisions can also have a major effect, such as changes to the area plan, discount rates and values of time. For rail schemes, inaccuracies are often blamed on uncertain measurement of passenger journeys and deliberately slanted forecasts by over-optimistic promoters.
Reference class forecasting
Reference class forecasting
Reference class forecasting is the method of predicting the future, through looking at similar past situations and their outcomes.Reference class forcasting predicts the outcome of a planned action based on actual outcomes in a reference class of similar actions to that being forecast. The theories...
was developed to identify inaccuracy in forecasting with its first application being the Edinburgh tram system in 2004.
See also
- Air traffic controlAir traffic controlAir traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
- Journal of Transport and Land UseJournal of Transport and Land UseThe Journal of Transport and Land Use is an open access peer-reviewed academic journal covering the interaction of transport and land use that was established in 2008. As of August 2011, it is the official journal of the World Society on Transport and Land Use Research...
- Optimism biasOptimism biasOptimism bias is the demonstrated systematic tendency for people to be overly optimistic about the outcome of planned actions. This includes over-estimating the likelihood of positive events and under-estimating the likelihood of negative events. Along with the illusion of control and illusory...
- Reference class forecastingReference class forecastingReference class forecasting is the method of predicting the future, through looking at similar past situations and their outcomes.Reference class forcasting predicts the outcome of a planned action based on actual outcomes in a reference class of similar actions to that being forecast. The theories...
- Road traffic control
- Traffic congestion: Reconstruction with Kerner’s three-phase theoryTraffic congestion: Reconstruction with Kerner’s three-phase theoryVehicular traffic can be either free or congested. Traffic occurs in time and space, i.e., it is a spatiotemporal process. However, usually traffic can be measured only at some road locations...
External links
- Transportation Systems Analysis Model - TSAM is a nationwide transportation planning model to forecast intercity travel behavior in the United States.