The appellants also contend that Section 19(E)(2) violates their right to equal protection of the laws,
U.S.Const., Amend. XIV, by providing for different and unequal treatment of nonconforming uses than do other code provisions. Signs which extend more than three feet into or over public property must be abated by September 1, 1975. Section 19(E)(2). Signs which are nonconforming for failure to comply with other regulations, such as size, area, or height, are permitted to remain indefinitely, as long as there is no abandonment, damage, destruction, or obsolescence. Section 19(E)(4).