Recent updates to the tools used by the Transportation Planning Board to forecast future travel patterns in metropolitan Washington will provide more accurate predictions of where, when, and how the millions of current and future residents of the region will travel in coming decades.
The changes, which took nearly four years to develop and were approved by the TPB in November 2011, will give planners and decision-makers better information to use to identify highway, transit, and other transportation infrastructure investments that will be most needed in the future in order to meet the region's growing -- and changing -- travel needs. They will also aid in monitoring the region's progress toward meeting federal air quality standards for smog-forming pollutants.
Compared to the previous version, the TPB's new Version 2.3 Travel Demand Model divides the region into many more traffic analysis zones, or TAZs, which serve as theoretical "origins" and "destinations" for trips the model predicts. Increasing the number of TAZs from 2,191 in the old version of the model to 3,722 in the new version -- an increase of 70% -- will provide a finer level of detail and enhance the model's ability to forecast more complex travel patterns. For instance, the model will now be able to better forecast trips made by non-motorized travel modes like bicycling and walking, shorter-distance trips that a model with larger TAZs and less detail might have difficulty capturing.
The other major improvement included in the Version 2.3 update allows the model to predict the particular modes that travelers use when they make trips by transit -- whether commuter rail, Metrorail, bus, or a combination -- and provide a better idea of the routes on which those trips are made. The previous version of the model was only able to estimate the total number of transit trips between a given origin and destination, not to predict the mode or route that travelers used. This change will allow planners and decision-makers to see more clearly how demand for transit services is distributed throughout the region, and where improvements need to be made to accommodate future growth or to take greater advantage of existing infrastructure.
These major changes to the model were accompanied by significant updates to the demographic and travel behavior information on which the models build their forecasts. Results from the TPB's most recent Household Travel Survey, which took place in 2007 and 2008, replace information on household travel patterns that was last collected on a regional scale in 1994. For a representative sample of households -- approximately 9,700 total households in the Washington region and 1,700 in the Baltimore area -- the survey captured information on such things as the number of cars a household owns or has access to, how many trips those living in the household make each day, and when, for what purpose, and by what travel modes those trips are made.
The models also rely on other details about the relative availability and attractiveness of various travel modes -- the factors that might lead a traveler to choose one mode over another -- in predicting whether people will choose to drive, take transit, or bike or walk for a given trip, and which route they will choose to take.
Trip costs and travel time are of particular importance in determining what modes and routes people choose, as are a household's proximity to transit services and whether it has access to a personal vehicle. Trip costs include transit fares, gas prices, parking costs, and tolls, while travel time depends not only on the time actually spent in the car or on a transit vehicle, but also time spent parking and "unparking" one's car, waiting for transit or transferring between routes, or the amount of time spent walking from a parking space or transit stop to one's final destination.
Together, the updates made in the Version 2.3 Travel Demand Model were designed to reflect changes that have occurred in the Washington region's development patterns, the availability and attractiveness of different transportation options, and how people make transportation decisions.
The culmination of several years of effort by TPB staff and the region's various stakeholder agencies, the TPB's newly-updated travel forecasting tools will provide planners, decision-makers, and the interested public with more accurate and detailed information about the region's future transportation needs and how best to address them.