A Study of Inclement Weather Impacts on Freeway Free-Flow Speed


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Virginia Tech


The research presented in this thesis attempts to investigate the impact of detector failure frequency and failure duration on the accuracy of loop detector speed, flow, and density measurements using a Monte Carlo simulation approach. The inputs to the model are the frequency of failures and failure duration. Several regression models were developed to relate loop detector accuracy to detector failure data. The results showed that the models were consistent and similar for the same location with an R square that ranged between 86% and 94% for all models and in comparing two locations, the differences between the regression models were minor except for the flow model errors, the location had the same trend but the magnitude of the flow RMSE increased by 7.5 to 15%.

The second part of the research effort attempts to quantify the impact of inclement weather (precipitation and visibility) on traffic stream free-flow speeds along freeway sections. The analysis is conducted using weather (precipitation and visibility) and loop detector data (speed) obtained from Baltimore, Minneapolis/St. Paul, and Seattle, US. The results demonstrate that visibility alone has a minimum impact on free-flow speed with reductions in the range of 1 to 3%. These reductions only appear as the visibility level falls below 1.2 km. The study demonstrates that the impact of snow is more significant than that of rain for similar intensity levels. Reductions caused by rain are in the neighborhood of 2 to 5% depending on the precipitation intensity while reductions caused by snow are in the neighborhood of 6 to 20%. With regards to freezing rain, higher reductions in free-flow speed were observed when compared to rain and snow. Specifically, the free-flow speed was reduced by 14% at the onset of freezing rain precipitation with a maximum decrease of 27% at freezing rain intensity of about 0.53 cm/h for Baltimore and as the case of Seattle the reduction was found to be constant with 31%.

Finally, the paper derives free-flow speed reduction factors that vary as a function of the precipitation type and intensity level. These reduction factors can be incorporated within the Highway Capacity Manual's procedures.



Weather, Free-Flow Speed, Loop Detectors, Speed Adjustment Factors