Short note: thirtieth highest hourly volume

The thirtieth highest hourly volume (30 HV) is the hourly traffic volume that is exceeded in only 29 hours of the year. It is obtained by counting the traffic in every hour of the year (8,760 hours), arranging the hourly volumes in descending order, and taking the 30th value.

 Hourly volume
 (% of AADT)
 |*
 | *
 |   *
 |      *___        <- knee at about the 30th hour
 |          `--.___
 +--------------------> hours (ranked)
   1   30    100   ...  8760

Why it is used

• On a graph of ranked hourly volume (as a percentage of AADT) against rank, the curve is steep up to about the 30th hour and then flattens (the "knee"). Designing for an hour beyond the knee saves very little, because the volumes there are nearly the same, while designing for one of the highest hours (for example the 1st or 5th) is uneconomical because the road would be under-used almost all the time.
• Designing for the 30th hour means the road is congested for fewer than 30 hours in a year, a reasonable economic balance.
• For rural roads the 30th highest hourly volume is taken as the design hourly volume (DHV). It is commonly about 8-15 percent of the AADT (the factor $K$), and the capacity (design service volume) of the road is fixed to carry it:

$$DHV = K \times AADT$$

• It is also used in highway planning in Nepal (Nepal Road Standard 2070) and in IRC practice (IRC:SP:19) to fix the number of lanes, while for urban roads the peak hour volume and PHF are used instead.