Evaluation of free flow speeds on interrupted flow facilities.
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Evaluation of free flow speeds on interrupted flow facilities.

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  • English

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      The efficacy of the Florida Department of Transportation (FDOT) simple model of predicting segment free flow speed by adding 5 miles per hour (mph) to the posted speed limit was compared to the performance of the new 2010 Highway Capacity Manual (HCM 2010) procedure which predicts free flow speed using posted speed limit and eight additional variables, i.e., the proportion of segment length with restrictive median, the proportion of segment with curb on the right-hand side, the number of access point approaches on the right side in the subject direction of travel, the number of access point approaches on the right side in the opposing direction of travel, the segment length, the width of the signalized intersection, the number of through lanes, and the distance between intersections. One-year speed data from 84 traffic monitoring sites located on interrupted flow facilities with speed limit ranging from 25 mph to 55 mph were used in the study. In addition, 3-day speed data were collected from 20 sites in the City of Tallahassee. Field mean free flow speed was determined for each analysis segment as well as the above geometric and traffic attributes required by HCM 2010 to predict free flow speed. The analyses were conducted separately for major arterial segments and for minor arterial segments. The comparison of the performance of the HCM 2010 and the FDOT free flow speed prediction models using root mean square error (RMSE) and the coefficient of determination (R-squared) showed that the FDOT simple formula of determining free flow speed performed better than HCM 2010 procedure which requires nine input variables to predict free flow speed. In both principal arterials and minor arterials, the HCM 2010 methodology under-predicted free flow speed when field estimated free-flow speed was higher than 40 mph. Consequently, the use of free flow speed predicted by the HCM 2010 model in level of service (LOS) analysis in some cases produced lower LOS compared to the use of field measured free flow speed.
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