Metropolitan Travel Forecasting Current Practice and Future Direction -- Special Report 288 (2007) / Chapter Skim
Currently Skimming:
4 Current State of the Practice
4 Current State of the Practice
Pages 46-64
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From page 46... ...
that provide travel forecasting services for multiple MPOs] , designed to obtain more detailed information.
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From page 47... ...
WEB-BASED SURVEY The web-based survey was structured to obtain information that would quantifiably describe the travel forecasting procedures of a broad sample of MPOs. The express purpose was to identify the state of the practice in travel demand modeling on the basis of the current practices of regional MPOs.
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From page 48... ...
Following is a summary of the basic steps of the travel modeling and forecasting process as it is currently practiced at most MPOs, based on the survey results. Input Data Agencies make use of extensive input data in developing travel models and preparing travel forecasts.
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From page 49... ...
The committee's web-based survey found that almost all MPOs require forecasts of population, households, and employment as input to their travel forecasting process. About half also forecast one or more of the following: household size, automobile ownership, and income.
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All of the large MPOs that reported having local bus service include at least three quarters of the local routes in their network. In contrast, more than 60 per cent of the small MPOs and 20 percent of the medium MPOs that reported having local bus service include less than three-quarters of local service miles in their network.
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From page 51... ...
Mode Choice Mode choice is the allocation of trips between automobiles and public transit. Within automobile travel, there is further allocation between drivers and passengers; within public transit, there may be allocation among local bus, express bus, and various rail options.
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From page 52... ...
Feedback Travel times are typically required to estimate trip distribution and mode choice; however, travel times depend on the level of congestion on routes in the network, which is determined only after trip assignment has been com pleted. Once congested travel times have been determined by the assignment process, these adjusted travel times should ideally then be fed back through the distribution, mode-choice, and assignment processes to produce more realistic estimates of travel.
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From page 53... ...
These detailed emission model inputs are not usually travel model outputs and so must be postprocessed after the model has been run. Commercial and Freight Travel The treatment of commercial and freight travel is one area in which most travel forecasting models need substantial improvement.
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23% FIGURE 4-2 MPOs considering activity- or tour-based models. MPO INTERVIEWS The committee's in-depth data gathering, including interviews of key MPO staff and supplemental written documentation provided by selected MPOs, offers insights beyond those obtainable from the mere tabulation of survey data.
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From page 55... ...
Worth, Council of Texas Governmentsa Pikes Peak Area Colorado Springs, Council of Colorado Governments Regional Las Vegas, Nevada Transportation Commission of Southern Nevada a Agency was not interviewed in person but provided answers to the interview questions in written form. some indication that they were or had been engaged in developing or applying procedures that might be considered as advancing the state of the practice, were active in organizations such as AMPO, or had developed or applied travel forecasting models for multiple MPOs within a state.
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From page 56... ...
• Model calibration: After the model has been estimated, it is calibrated so that predicted travel accords with observed travel on highway and transit networks. • Model validation: After the model has been estimated and calibrated, it is validated to test its ability to predict future behavior.
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From page 57... ...
The agency typical of this group of MPOs has two or three staff involved in travel forecasting and spends $150,000 to $200,000 annually on model application. Another study found that MPOs with a population of less than 500,000 have an average of one full-time travel modeler on staff, while larger agencies average three full-time modelers.
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From page 58... ...
In addition, some agencies appear to be interested in developing more effective truck models and special generator models.1 Obstacles to Improvement Agencies interviewed cited a desire for tangible evidence that new procedures perceived as more complex or requiring significantly greater effort for devel opment and application would yield forecasts notably better than those pro duced with currently accepted procedures. Other factors cited as impeding the adoption of advanced techniques were the unavailability of vendor-supplied software needed for implementation, a lack of sufficient staff to apply the new techniques, the difficulty of finding staff versed in the development and appli cation of the techniques, and insufficient funds for the purpose.
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From page 59... ...
At the other end of the spectrum is a rapidly growing metropolitan area that is not in attainment of air quality standards, has severe congestion, and is planning to apply dynamic tolling for high-occupancy travel lanes on which there will also be bus rapid transit. In such an area, it will be desirable to have a travel forecasting process that (a)
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Lowry for Pittsburgh. Such models estimate the location and scale of (a)
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From page 61... ...
As many as nine trip purposes are currently used in MPO models; smaller MPOs are more likely to use fewer purposes. • Common practice: Trip distribution -- the process of determining the number of trips between each pair of zones -- is accomplished primarily with a gravity model.
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From page 62... ...
• Common practice: Mode choice is the allocation of trips between auto mobiles and public transit. Within automobile travel, there is allocation between drivers and passengers; within public transit, there may be allocation among local bus, express bus, and various rail options.
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From page 63... ...
MPO staffs recognize the limitations of their current forecasting procedures. Yet the agencies interviewed reported the following barriers to implementing advanced modeling practices: • A lack of tangible evidence that new procedures would yield forecasts notably better than those produced with currently accepted procedures.
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From page 64... ...
This chapter has presented information on the current state of the prac tice in metropolitan travel forecasting, including common practice, variations in practice, areas needing improvement, and reported barriers to improve ment. The next chapter reviews in greater detail the shortcomings of current forecasting models.
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