Is it legal to ride a dirt bike on public road but is not a main road?

It depends on your state laws. In states that it is legal, the dirt bike must be certified to be allowed on to any public road (including a main road) using the VIN number, and then must have a street legal kit installed, as well as getting it licensed. The street legal kit requires a headlight, brake light, horn, and a rear-view mirror. An optional inclusion would be turn signals, if you don't wish to use hand signals. It is also suggested that you use street tires.