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From page 1...
... S-1 Introduction Traffic forecasts are projections of future traffic conditions on existing or proposed roadways. Traffic forecasts are used to inform important decisions about transportation projects, including the selection of which projects to build and certain design elements of those projects.
From page 2...
... S-2 Traffic Forecasting Accuracy Assessment Research improve forecasting practice. In contrast, this study makes specific recommendations by which state departments of transportation (DOTs)
From page 3...
... Summary S-3 A third question addressed in this research is, "How can forecasting practice be improved? " To address this question, the project team conducted a workshop with traffic forecasting practitioners in which the lessons learned from this research could be reviewed and considered.
From page 4...
... S-4 Traffic Forecasting Accuracy Assessment Research Using this formulation, negative values indicate outcomes lower than forecast, and positive values indicate outcomes higher than forecast. The formulation appealed to the research team because it expresses the error as a function of the forecast, which always is known earlier than traffic counts.
From page 5...
... Summary S-5 counted traffic below 17,898. Discounting other variables, for a forecast volume of 30,000 the range 45,578 to 17,898 will capture the likely volumes experienced by 90% of the projects examined.
From page 6...
... S-6 Traffic Forecasting Accuracy Assessment Research of years between when the forecast is made and when the project opens. In this study, the quantile regression results showed that the median, 80th percentile, and 95th percentile estimates increase with an increase in this variable, but that the 5th and 20th percentile estimates either stay flat or increase by a smaller amount.
From page 7...
... Summary S-7 • Central Artery Tunnel, Boston, Massachusetts; • Cynthiana Bypass, Cynthiana, Kentucky; • South Bay Expressway, San Diego, California; and • US-41 (later renamed I-41) , Brown County, Wisconsin.
From page 8...
... S-8 Traffic Forecasting Accuracy Assessment Research 4. Assumptions about external traffic and travel speed also affect traffic forecasts.
From page 9...
... Summary S-9 forecasting by reporting the forecast in terms of a range. If an actual future traffic count at the low or high end of the range would not change the decision, the decision makers can safely proceed with little worry about the risk of an inaccurate forecast.
From page 10...
... S-10 Traffic Forecasting Accuracy Assessment Research Recommendation 3: Periodically report the accuracy of forecasts relative to observed data. The project team recommends that agencies responsible for producing traffic forecasts periodically report the accuracy of their forecasts relative to the outcomes measured when the roads are in service.
From page 11...
... Summary S-11 Why Should Transportation Agencies Implement These Recommendations? Conscientious forecasters strive for objectivity, but this effort does not necessarily ensure that their forecasts are accurate, nor does it ensure that their forecasts are viewed as credible in the eyes of decision makers or the public.
From page 12...
... S-12 Traffic Forecasting Accuracy Assessment Research • An Excel file containing working versions of the deep dive assessment tables referenced in Appendix C; and • A PowerPoint presentation file that summarizes key findings from this research. Using the links provided in Part III, Appendix A, agencies also can access online software (called Forecast Cards)

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