The design hourly volume i.e., DHV is generally the 30th highest hourly volume for the design year. The design year is commonly considered as 20 years from the time of completion of construction.
In the case of two-lane rural highways, the design hourly volume is the total traffic in both directions of travel.
Some adjustment of design hourly volume can be appropriate for situations involving high seasonal fluctuations in average daily traffic (ADT)
Knowledge of the directional distribution during the design hour (DDHV) has more importance for design.
DHV and DDHV can be computed by the application of conversion factors to ADT. The % of ADT occurring in design hour (k) can be used so as to convert ADT to DHV as shown in the following expression.
DHV = (K) (ADT)
Percentage of the design hourly volume which is in the definite direction of travel. D and K are both used to convert ADT to DDHV as follows,
DDHV = (K) (D) (ADT)
where, D = directional distribution
K = percentage of ADT representing the 30th highest hourly volume in the design year
Traffic should be more equally divided by the direction near the center of an urban area or on loop facilities. D factors if 60% to 70% frequently occur for other facilities.
K factors generally range from 12% to 18% for typical main rural highways. K - factors are typically ranging from 8% to 12% which is lower in percentage for urban facilities.
The 30th highest hourly volume is the hourly volume which can be exceeded only 29 times in a year and all other hourly volumes of the year are less than this value. There will be congestion only during 29 hours in the year and hence, the 30th highest hourly volume is generally considered as the hourly volume for design.