Hourly traffic volume means the volume of traffic of a particular hour. So for a day, there will be 24 hourly traffic volumes. And for a year there will be 365×24 hourly volumes. The hourly volume which is considered for design purpose is called design hourly traffic volume. This value should be such that traffic volume at most of the time will be less than this.

The

**maximum hourly volume**or the**peak hourly volume**(i.e. the highest of all hourly volumes) of the year should be the obvious choice for this. But it is found to be too high and hence unnecessary to be considered for design as it's not an economical choice.
The

**annual average hourly volume**(**AAHV**) that is the average of all the hourly volumes of a year is not sufficient for the design purpose as there will be a considerable amount of time at which the traffic volume will be more than this value. If the road will be designed for AAHV there will be severe congestion during a considerable period of the year.
So, the

**design hourly traffic volume**should be between these two values and should be satisfactory from the consideration of the facility as well as the cost. "**Thirtieth highest hourly traffic volume**'' is found to be satisfactory while considering both facility and economic point of view. Only 29 times in a year traffic volume will exceed this value that means there will be congestion only during 29 hours in the year and which is considered reasonable. The point to note that the cost will be much lesser when compared to the peak hourly volume. Thus the 30^{th}highest hourly volume is generally adopted as the design hourly volume for the purpose of roadway facility design.
To determine the

In Y-axis the hourly volumes are presented in terms of the percentage of

**30**a^{th}highest hourly volume**graph**as shown in the figure is plotted.**average daily traffic**(**ADT**). In X-axis the number of hours in the year having higher traffic volume than the corresponding hour in the Y-axis is presented. If (x_{1},y_{1}) is a point on the graph, y_{1}is (hourly volume×100)/ADT of a particular hour and x_{1}is the number of y values greater than y_{1. }From the value of y corresponding to x equals 30 the 30^{th}highest hourly volume or design hourly traffic volume can be determined.
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