Perhaps this is a bit elementary but you need to:
1. Convert your velocity to pressure via standard formulations for such.
2. Using the pressure apply it to the pole and anything attached to the pole, like signs,
3. Calculate the required section modulus by using the age old formula, M/S should be less than the allowable tensile stress of the steel used for the pole. M, of course, is the moment resulting from the resultant of the uniform pressure multiplied by the distance from the applied resultant load to the distance to the ground level where the pole is embedded.
Once you're done with that you'll have a preliminary idea of what you need in about 5 or 10 minutes. If that result is ok with your expectations, you'll need to check the governing codes to make sure the steel section is of sufficient thickness and diameter to meet tension and buckling constraints. You'll also need to determine how deep to embed the steel pole to achieve fixity.
I hope this helps. You probably need to check with an experienced structural engineer.
Regards,
Qshake
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