Oversaturated conditions occur when the demand for the facility exceeds the capacity. These conditions result in a significant drop in speeds and flow rate through a point or segment, depending on the severity of saturation. The HCM defines conditions as oversaturated when any of the following conditions are satisfied:

“(a) the arrival flow rate exceeds the capacity of a point or segment, (b) a queue created from a prior breakdown of a facility has not yet dissipated, or (c) traffic flow is affected by downstream conditions.”

For uninterrupted flow, oversaturated conditions result from a bottleneck on the facility. During oversaturated conditions, queues extend upstream from the point of the bottleneck.

It is recommended that traffic counts collected for oversaturated conditions span a time interval greater than just the peak hour or peak period. It is suggested that field observations are conducted, along with segment traffic counts, to determine the overall peak period and the shouldering effect of traffic or queue build up and dissipation. A shouldering effect occurs during a peak period when a facility is oversaturated and cannot process any additional demand, causing the peak period to extend longer than an hour. The effect can occur on both ends of the peak hour during a peak period, where the demand for the facility is higher than the capacity, allowing only a portion of the traffic volumes to process through the facility until the demand falls below capacity.

For oversaturated and bottleneck conditions, it is suggested that counts are started at least one hour before congestion starts and end one hour after congestion has been cleared. Similar to TMCs, oversaturated counts are collected on a typical weekday and for a minimum of two consecutive days, based on coordination with the TxDOT project manager. Depending on how congested the study area is, collect counts at least one intersection or interchange upstream of the study area.

In addition to traffic counts, it is recommended that queue data is collected for oversaturated conditions, depending on the type of analysis being performed. See

Section 2.2.4 - Queue Data

for calculating demand in oversaturated conditions.

Recommended oversaturated counts collection method: