FDOT has freeware for signal poles on a single pile.
Respect footings, the question is more than anything to meet the allowable stresses.
I understand the push for economy that drives everyone, but for single items like this is scarcely worth. A proper training should produce designers able to do this design without any doubt. What we can't do at one forum like this is to preovide a complete engineering design for some requested items.
Respect bolts and so on, usually anchor bolts are embedded to the footing, then the pole is leveled to stay vertical, and the nuts locked to this position. A plate or more complex design ensures proper transfer of forces between pole and foundation through the bolts.
Wind is a dynamical solicitation and many bolts anchoring signal poles with big panels get broken. In some codes, friction bolts are required never permitting the interface to lose contact are mandated. In all cases, to reduce the stress range in tension will enhance the (fatigue) life of the bolts. To use ductile nuts will also help against their failure in fatigue.
How long was the sign in its original location? If it has been there for quite some time, I would copy the original design if available. No use trying to reinvent the wheel.
titled: "Engineering Sign Structures," by Benjamin Jones. Everything you need to know regarding sign structures and their foundations can be found in this text.
Are you using a new pole? If existing does it already have a base plate? Does whoever is putting up the sign have drilling equipment? Is it possible to drill at the new location (rock below topsoil)? Is there any limitations on footing size at the new location? Can the pole be embedded as well to avoid base plate all together?
Embedded is often more economical with the larger billboard signs (your case may be different). Reference UBC to help with embedment requirements.
Footing should be designed as a short pile, using the passive pressure against the side of the pier to resist the overturning. (I believe it the Brom's method) This calculation is similar to that used for light pole designs. Soil conditions are important, as is compaction around the pier, but 24" diameter sounds in the ballpark, with embedment in the 6ft to 7ft range. Enercalc has this calculation in its software package (as does DCALC, for all you bridge designers).
AASHTO Maual for Highay Signs and Luminaires is a good reference for the design of sign structures. It provides the loading and design limitations on Extended Projected Area (EPA) of the poles. As far as the foundation is concerned, AASHTO also provides the specs for the design of drilled shaft foundation (mentioned as short pile by Zulak). Alternatively, spread footing can be designed using the usual design methods. However, drilled shaft is more economical and easy to construct.
