United States Department of Transportation
i
Estimation of capacities on Florida freeways.
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2014-09-01
[PDF-2.73 MB]
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Abstract:Current capacity estimates within Florida’s travel time reliability tools rely on the Highway Capacity Manual (HCM 2010) to
estimate capacity under various conditions. Field measurements show that the capacities of Florida freeways are noticeably
lower than the values recommended in the HCM 2010, by an order of a few hundred vehicles. In addition, recent research has
shown that maximum freeway throughput may differ between undersaturated and oversaturated conditions, and this is not
acknowledged in the HCM 2010.
The main objective of this research was to collect field data at several urban and rural freeway and multilane locations in Florida
in order to measure capacity flows and to provide recommended capacity values before and after the initiation of oversaturation.
The research team obtained data at urban and rural freeways and multilane highway segments across Florida. The urban
freeway data were obtained through the Statewide Transportation Engineering Warehouse for Archived Regional Data
(STEWARD) at various types of bottlenecks including merge junctions, weaving segments, as well as geometric bottlenecks
(lane drops), while the rural freeway and multilane highway data were obtained from the permanent count stations of FDOT.
Incidents and weather data were also obtained to ensure that the final datasets included capacity observations due to excess
demand and not due to random events such as incidents or bad weather.
Various capacity measures were investigated, and it is recommended to define pre-breakdown capacity as the 85th percentile of
the 15-min average pre-breakdown flow and the post-breakdown capacity as the average discharge flow. A clear drop in
throughput between pre-breakdown and discharge capacity values was observed. Recommendations on capacity values as a
function of the number of lanes and the segment type for both urban and rural locations are offered. This research also proposes
revised density thresholds for defining Level of Service on various types of segments consistent with the recommended capacity
values.
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