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I'm planning to attend a Californian university as a visiting scholar (not paid by the university) on a J-1 visa, for a total of four weeks; a colleague of mine will do the same, but for 12 weeks. I've been told (personally, ie. no citable sources) that, if you're staying on a J-1, you're effectively a California resident for the time you're there, and, therefore, need a Californian driver's license.

I've read up on this and found this DMV page that states:

If you are a visitor in California over 18 and have a valid driver
license from your home state or country, you may drive in this state
without getting a California driver license as long as your home state
license remains valid.

If you become a California resident, you must get a California driver
license within 10 days. Residency is established by voting in a
California election, paying resident tuition, filing for a homeowner's
property tax exemption, or any other privilege or benefit not
ordinarily extended to nonresidents.

I'm not planning to vote in any elections, nor will I own a home there, but I'm not sure what would constitute "privileges or benefits not ordinarily extended to nonresidents".

Do we need to get driver's licenses in California, or can we use our foreign licenses? How about day trips to Arizona or Nevada?

Note: this seems to be related to What are the residence requirements to use a foreign driving licence in California?, but my question is specifically about staying there on a J-1.

The line between resident and nonresident for California driving purposes is murky. As you have read, certain actions mean you are probably a resident, but this is not a complete list, so just because you have done none of those things doesn't mean you are a nonresident.

Here is what the law says:

CVC 12502(a)(1):

(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.

CVC 12505(a)(1):

(a) (1) For purposes of this division only and notwithstanding Section
516, 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.

So basically it boils down to whether California is your "domicile", but the definition of that is also somewhat subjective.

Depends if you are paying resident tuition or nonresident tuition. If paying nonresident then as long as your home state or country's driving licence is valid you are fine using that.

New California Residents

want to drive in California, you must apply for a California DL within
10 days. Residency is established in a variety of ways, including the
following:

• Being registered to vote in California elections.


• Paying resident tuition at a California college or university.


• Filing for a home owner’s property tax exemption.


• Receiving any other privilege or benefit not ordinarily extended to nonresidents.

Adults Visiting California

from their home state or country may drive in California without
getting a California DL as long as their home state or country DL is
valid.

Link to State of California Department of Motor Vehicles