Date of Award

Spring 2009

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Civil, Construction, and Environmental Engineering


Urban and suburban population increase causes an increase in traffic demand on the current highway system which results in the problem of traffic congestion. This study examined the problem of freeway congestion and provided some insights that can help in improving the understanding of the freeway congestion mechanism. Congested locations within the Milwaukee, WI freeway system were used in the analysis. The research presents some important characteristics at congested locations such as insights into the breakdown mechanism, the probabilistic nature of two capacity regimes (capacity under free-flow and under queue discharge flow conditions), and the weather impact on these two freeway capacity regimes.

Freeway capacity was found to be a stochastic event adequately described according to the shifted Weibull distribution. The two above-mentioned capacity regimes were found to be significantly different and the recommendation was made to treat them as separate entities. Freeway breakdowns were found to occur under a high lane utilization. A capacity drop was typically observed in this case. When lanes were not evenly utilized at the onset of a breakdown event, it was possible that capacity under congested conditions could exceed capacity under free-flow conditions.

The presence of fog, rain, and snow were found to reduce freeway capacity; throughput reduction was steeper for more severe weather conditions. Queue discharge flow sensitivity to weather variables was found to be considerably higher compared with the free-flow capacity. Increasing the ramp volume share of the total flow at the merge influence area was found to reduce the merge influence area capacity and increase the breakdown duration considerably.



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