The Zoning Hearing Board is empowered to grant special exceptions in order to prevent or to lessen such practical difficulties and unnecessary physical or economic hardships inconsistent with the objectives of this Part as would result from a strict or literal interpretation and enforcement of certain of the regulations herein prescribed. A practical difficulty or unnecessary physical hardship may result from the size, shape or dimensions of a site or the locations of existing structures thereon; from geographic, topographic or other physical conditions on the site or in the immediate vicinity; from population densities, street locations or traffic conditions in the immediate vicinity; or from the location or height of the sign in question. The power to grant a special exception does not extend to an economic hardship related to the cost, size or location of a new sign, or to the convenience of the applicant; nor is it intended to extend to the convenience of regional or national businesses that wish to use a standard sign, when it does not conform to the provisions of this chapter.