First, How did you collect and count your traffic volume data? the traffic volume is dependent on the seasonal and hourly factors. So, you should provide the detail information, how you collect it, at what time and season did you collect it and what kinds of counting mechanisms did you use?(was it manual? or was it automatic sensor counting mechanisms?). After that, you will have the techniques to calculate Average daily traffic(ADT) and average annual daily traffic(AADT).

There are some typical coefficients that could be used to predict AADT from Peak hour volume and vice versa. The general equation is:

DDHV = AADT*K*D where DDHV= directional design hour volume (your 15-minute count times 4 times)

K = proportion of daily traffic occurring during the peak hour

D= proportion of peak hour traffic traveling in the peak direction of flow

There are some recommended values for K and D depending on the type of road (rural, suburban, urban, etc.)

You can check "traffic engineering" book by Roess, Prassas, and McShane for these values.

Dear

You need to do counting for 24 hours then you check what is the percentage of the calculated 15 minutes volume to that of the 24 hours volume.

Agreed with @ Saad Sarsam

Laxman Singh Bisht for this calculation you need peek hour factor for the locations that you have 15 minutes traffic volume count. If you do not have peak hour factor you can make some assumptions (for example you can use 0.25 for PHF). After that you can calculate DDHV (multiply 15 minutes traffic volume values with 4) and you can use the formula defined by Amara عمارة Loulizi الوليزي.