(1) A person’s employment is principally localized in this or another state when: His employer has a place of business in this or such other state and he regularly works at or from such place of business; or
(2) He is domiciled and spends a substantial part of his working time in the service of his employer in this or such other state.
(3) An employee whose duties require him to travel regularly in the service of his employer in this and one or more other states may, by written agreement with his employer, provide that his employment is principally localized in this or another such state, and, unless such other state refuses jurisdiction, such agreement shall be given effect under this law.