A lei relevante é o Código de Veículo da Califórnia §12502 e §12505.
De acordo com §12502,
(a) The following persons may operate a motor vehicle in this state without obtaining a driver’s license under this code:
1. A nonresident over the age of 18 years having in his or her immediate possession a valid driver’s license issued by a foreign jurisdiction of which he or she is a resident, except as provided in Section 12505.
2. ...
De acordo com §12505,
O que tudo isso significa é que, sim, se você tem em sua posse uma carteira de motorista emitida pelo seu país de residência, e legalmente permite que você opere uma motocicleta, você pode usar essa carteira de motorista estrangeira na Califórnia para operar uma motocicleta.[...] residency shall be determined as a person’s state of domicile. "State of domicile" means the state where a person has his or her true, fixed, and permanent home and principal residence and to which he or she has manifested the intention of returning whenever he or she is absent. Prima facie evidence of residency for driver’s licensing purposes includes, but is not limited to, the following:
(A) Address where registered to vote.
(B) Payment of resident tuition at a public institution of higher education.
(C) Filing a homeowner’s property tax exemption.
(D) Other acts, occurrences, or events that indicate presence in the state is more than temporary or transient.
[...] (c) Any person entitled to an exemption under Section 12502, 12503, or 12504 may operate a motor vehicle in this state for not to exceed 10 days from the date he or she establishes residence in this state
Há também um writeup decente por não um advogado em Califórnia Driving: A Survival Guide