Depending on the state you are operating in, you may be required to obtain a USDOT number even if you only operate intrastate and your cargo or vehicle never crosses state lines. Some states have their own intrastate regulations that require carriers to obtain a USDOT number, while others may not. It is recommended that you check with your state's Department of Transportation or responsible state agency to determine if you need a USDOT number. Additionally, even if you are not required to obtain a USDOT number, you may still need to comply with other state and federal regulations, such as safety and insurance requirements. 

You may also refer to the FMCSA Registration Page.