The Congestion Assessment was done using either of two methods, depending on the roadway type. Average peak hour vehicle speed was utilized on the freeways and expressways to monitor congestion levels. Traffic counts were used to evaluate congestion levels on signalized arterial roadways. Vehicle probe speed data was used to determine the Travel Time Index (TTI)* on the majority of freeways and expressways, and the corresponding values were used to grade the congestion levels on those roadways. On the remaining roads (i.e. signalized arterial roadways), congestion was based on levels of service (LOS) calculated using 2000 Highway Capacity Manual (HCM) procedures. This methodology utilizes weekday peak hour volumes to determine the effects of demand on the roadway
Roadway congestion falls into four categories: Uncongested, Moderately Congested, Heavily Congested and Severely Congested. We classify the freeway and expressway congestion based on their TTI Values: Uncongested is less than 1.15, Moderate Congestion between 1.15 and 1.3, Heavy Congestion between 1.3 and 2 & Severe Congestion greater than 2.
The signalized arterial roadways are classified based on the LOS grade obtained from the HCM based calculations. LOS A, B & C represent Uncongested conditions, where as LOS D corresponds to Moderate Congestion and LOS E & F represent Heavy Congestion.
* TTI is the ratio of travel time during free flow to the same trip during traffic congestion. For example, a TTI of 1.5 means a trip taking 10 minutes in uncongested conditions will take one and half times as long, or 15 minutes, in congestion. Alternatively, TTI is the ratio of the free flow speed to the congested speed.
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