K factor (traffic engineering)

In transportation engineering, the K factor is defined as the proportion of annual average daily traffic occurring in an hour.[1] This factor is used for designing and analyzing the flow of traffic on highways. K factors must be calculated at a continuous count station, usually an "automatic traffic recorder", for a year before being determined. Usually this number is the proportion of "annual average daily traffic" (AADT) occurring at the 30th-highest hour of traffic density from the year's-worth of data. This 30th-highest hour of traffic is also known as "K30" or the "Design Hour Factor". This factor improves traffic forecasting, which in turn improves the ability of designers and engineers to plan for efficiency and serve the needs of this particular set of traffic.[2] Such forecasting includes the selection of pavement and inclusion of different geometric aspects of highway design, as well as the effects of lane closures and necessity of traffic lights.[3] Engineers have reached consensus on identify K30 as reaching a reasonable peak of activity before high outliers of traffic volume are used as determinative of overall patterns.[3] The K factor has three general characteristics:

• K generally decreases as AADT increases.
• K generally decreases as development density increases.
• K is generally highest near recreational facilities, next highest in rural and suburban areas, and lowest in urban areas.

Another notable proportions of K is the measure of K100 (the proportion of AADT occurring during the 100th highest hour of the design year). This proportion is also known as the Planning Analysis Hour Factor.[2]