The Annual Average Daily Traffic (AADT) obtained from the model or trend analysis provides a general indication of the travel demand for a transportation facility; however, it does not reflect the hourly variation of travel demand. If the design is based on AADT, there could be many hours of a day when the facility will fail. Traditionally, it is accepted that taking a peak-hour represents a balance between choosing a very short peak period [e.g., five (5) minutes] and choosing a very long peak period that will result in a long failure time. The volume corresponding to the peak hour is the design hour volume (DHV). However, the DHV does not capture the spatial variation in traffic demand between the two directions. The traffic in the peak direction represents the highest demand for travel and therefore is used to determine the number of lanes required to accommodate the traffic flow. Furthermore, considering the temporal variation of traffic within an hour, there may be time periods where the facility could still fail. One way to measure the sub-hour traffic variation is to calculate the Peak Hour Factor (PHF), which equals the hourly volume divided by the peak 15-minute traffic within the hour multiplied by four. The Directional Design Hour Volume (DDHV) and PHF are used together with heavy vehicle parameters to compute equivalent hourly flow rate. The flow rate is then used to estimate the Level of Service (LOS) for the facility.

As previously mentioned, DDHV can be obtained by multiplying AADT by K and D factors. DDHV can be used for design and traffic operational analysis for the design hour. In most cases, it is necessary to estimate directional peak hour volumes for AM and PM peak-periods for a typical day. If the design hour happens to coincide with either the AM or PM peak-hour of a typical day, and if AM and PM peak-hour happen to be the mirror image of each other, then no additional work is needed. If they do not mirror each other, the peak-period volumes for the AM and PM peak-periods need to be estimated separately. The directional peak-hour volumes can then be used to further estimate turning movement volumes for intersection operational analysis.