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Assessment of traffic impact studies on Oahu between 1976 and 2002

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Item Summary

Title:Assessment of traffic impact studies on Oahu between 1976 and 2002
Authors:Caroee, Maja
Keywords:traffic forecasting
civil engineering
Date Issued:May 2012
Publisher:[Honolulu] : [University of Hawaii at Manoa], [May 2012]
Abstract:Traffic forecasting is an essential part of project development. The impact that forecast traffic has on infrastructure planning and design is essential for project approval. Since population, employment and tourism are projected to grow by 17%-25% on Oahu over the next 25 years, the need for further development is clear. So the key question is: Can decision makers rely on project traffic forecasts?
Previous studies showed large inaccuracy in traffic forecasting, 50% of all megaprojects have inaccuracies larger than ±20%, and 25% of all megaprojects have inaccuracies larger than ±40%. In a U.S. study of roadways the inaccuracies were not as large, 72.4% of all road segments analyzed were within ±0.5% accuracy; the remaining 27.8%% were either over-or underestimated. These studies showed major problems with forecasting accuracy.
The analysis herein compared forecasted traffic levels from Traffic Impact Analysis Reports, available through the Hawaii Department of Health, to actual traffic volumes recorded by the Hawaii Department of Transportation to assess traffic forecasting accuracy on Oahu between 1976 and 2002. Information extracted from the EIS and TIAR included: year of EIS, consultant, type of project, location, movement, forecast horizon, forecasted traffic volumes and forecasting method.
This study focused on road and residential developments on Oahu and attempted to answer questions such as: Do forecasts show an accurate picture of future traffic demand, both related directly to a specific project and the affected region? Do forecasts provide a "picture perfect" view of future conditions in order to get the projects approved? Is traffic forecasting on Oahu conservative or optimistic? Do forecasts vary by type of development? What is the effect of the forecast horizon?
What is the effect of background growth extrapolations?
The analysis is split into three components: Illustrative analysis, quantitative analysis, and a three-way comparison among (1) Flyvbjerg's study of more than 200 megaprojects across the world, (2) a study conducted in Minnesota by Parthasarathi and Levinson on post-construction evaluation of traffic forecast accuracy, (3) and the results of this analysis.
The analysis of Oahu data shows a tendency towards overestimation of traffic forecasts. The average overestimation is 35% or more than one third of the forecast volume. Fourteen out of the 37 cases, or 38%, were underestimated: nine cases with an error of greater than-20%, two cases with an error of greater than-40% and three cases within the-60% to-40% range. The findings of this study differ greatly from the two studies it is compared to. The study of mega-projects world-wide shows a tendency towards overestimation, the study of traffic accuracy in Minnesota shows very accurate forecasting volumes.
There are several reasons for inaccuracy in traffic forecasting, such as inappropriate forecasting models, the assumptions and growth rates used as input into the models, the large uncertainties associated with green field developments, bias for or against projects and lack of an evaluation and adjustment process.
Description:M.S. University of Hawaii at Manoa 2012.
Includes bibliographical references.
Appears in Collections: M.S. - Civil and Environmental Engineering

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