Vehicles

It is LEGAL for any person, at least 18 years old and not a prohibited person as defined in 76-10-503 or 18 U.S.C. 922(g) to have a loaded handgun anywhere in a vehicle as long as it is their vehicle or they have the consent of the owner of the vehicle. The handgun can also be carried concealed on your body while you are in the vehicle even if you do not have a concealed firearm permit.
It is ILLEGAL to have a loaded rifle, shotgun or muzzle-loading rifle in a vehicle unless you have a concealed firearm permit and it is your vehicle or you have the consent from the owner of the vehicle.
It is ILLEGAL to have any firearm in a vehicle in any prohibited area under state and federal law such as a secure parking area of correctional facilities, mental health facilities, on any military bases and post offices.
See the parking lot law here.

• If you do not have a concealed firearm permit, when you exit the vehicle the handgun must be unloaded and open carry.
• If you leave a firearm in a vehicle while the vehicle is not occupied, the firearm must be locked securely in the motor vehicle or in a locked container attached to the motor vehicle and the firearm cannot be in plain view from the outside of the motor vehicle.

It is LEGAL for a nonresident traveling in or through Utah to have a firearm in their vehicle without any type of permit or license as long as the firearm is unloaded and securely encased (see laws below)
It is LEGAL for any lawful person to transport unloaded firearms in a vehicle while traveling through any state, during such transportation the firearm is unloaded (no ammunition in the firearm), and neither the firearm nor any ammunition being transported is readily accessible or is directly accessible from the passenger compartment of such transporting vehicle: Provided, That in the case of a vehicle without a compartment separate from the driver's compartment (pickup truck) the firearm or ammunition shall be contained in a locked container other than the glove compartment or console.
• For instance, an individual who is a resident of Utah is going on vacation to visit some friends and go shooting in Arizona. He is taking his AR15 with two 30 round magazines, 200 rounds of ammunition and a Glock 17 with three 17 round magazines containing hollow point ammunition. All of these items are legal to own and possess in both Utah and Arizona. During his road trip, he will be driving through Nevada and California, finally arriving in Arizona.
Well as many people already know, California is a highly regulated state when it comes to firearms. San Francisco has a ban on hollow point ammunition and the state as a whole has a ban on "assault rifles" and magazines that hold more than 10 rounds. This law allows you to have your "California illegal" firearms, ammunition and magazines in your vehicle as long as the firearms are unloaded.

Be aware that police may still arrest you even if you are following this legal Federal law

There is no legal requirement to identify yourself to a law enforcement officer.
See Duty To Notify Law Enforcement

For information about having a firearm in your vehicle in parking lots click here.

STATE LAWS

41-1a-102Definitions.
As used in this chapter:
(8) "Camper" means any structure designed, used, and maintained primarily to be mounted on or affixed to a motor vehicle that contains a floor and is designed to provide a mobile dwelling, sleeping place, commercial space, or facilities for human habitation or for camping.
(28) "Manufactured home" means a transportable factory built housing unit constructed on or after June 15, 1976, according to the Federal Home Construction and Safety Standards Act of 1974 (HUD Code), in one or more sections, which, in the traveling mode, is eight body feet or more in width or 40 body feet or more in length, or when erected on site, is 400 or more square feet, and which is built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation when connected to the required utilities, and includes the plumbing, heating, air-conditioning, and electrical systems.
(21) "Highway" or "street" means the entire width between property lines of every way or place of whatever nature when any part of it is open to the public, as a  matter of right, for purposes of vehicular traffic.
(30) "Mobile home" means a transportable factory built housing unit built prior to June 15, 1976, in accordance with a state mobile home code which existed prior to the Federal Manufactured Housing and Safety Standards Act (HUD Code).
(32) "Motorcycle" means:
(a) a motor vehicle having a saddle for the use of the rider and designed to travel on not more than three wheels in contact with the ground; or
(b) an autocycle.
(33) (a) "Motor vehicle" means a self-propelled vehicle intended primarily for use and operation on the highways.
(b) "Motor vehicle" does not include an off-highway vehicle.
(34) (a) "Nonresident" means a person who is not a resident of this state as defined by Section 41-1a-202, and who does not engage in intrastate business within this state and does not operate in that business any motor vehicle, trailer, or semitrailer within this state.
(b) A person who engages in intrastate business within this state and operates in that business any motor vehicle, trailer, or semitrailer in this state or who, even though engaging in interstate commerce, maintains any vehicle in this state as the home station of that vehicle is considered a resident of this state, insofar as that vehicle is concerned in administering this chapter.
(41) "Park model recreational vehicle" means a unit that:
(a) is designed and marketed as temporary living quarters for recreational, camping, travel, or seasonal use;
(b) is not permanently affixed to real property for use as a permanent dwelling;
(c) requires a special highway movement permit for transit; and
(d) is built on a single chassis mounted on wheels with a gross trailer area not exceeding 400 square feet in the setup mode.
(67) "Travel trailer," "camping trailer," or "fifth wheel trailer" means a portable vehicle without motive power, designed as a temporary dwelling for travel, recreational, or vacation use that does not require a special highway movement permit when drawn by a self-propelled motor vehicle.
(69) "Vehicle" includes a motor vehicle, trailer, semitrailer, off-highway vehicle, camper, park model recreational vehicle, manufactured home, and mobile home.

41-22-2. Definitions.
As used in this chapter:
(2) "All-terrain type I vehicle" means any motor vehicle 52 inches or less in width, having an unladen dry weight of 1,500 pounds or less, traveling on three or more low pressure tires, having a seat designed to be straddled by the operator, and designed for or capable of travel over unimproved terrain.
(3)(a) "All-terrain type II vehicle" means any other motor vehicle, not defined in Subsection (2), (11), or (22), designed for or capable of travel over unimproved terrain.
(b) "All-terrain type II vehicle" does not include golf carts, any vehicle designed to carry a person with a disability, any vehicle not specifically designed for recreational use, or farm tractors as defined under Section 41-1a-102.
(10) "Motorcycle" means every motor vehicle having a saddle for the use of the operator and designed to travel on not more than two tires.
(11) (a) "Motor vehicle" means every vehicle which is self-propelled.
(b) "Motor vehicle" includes an off-highway vehicle.
(13) "Off-highway vehicle" means any snowmobile, all-terrain type I vehicle, all-terrain type II vehicle, or motorcycle.
(22) "Street or highway" means the entire width between boundary lines of every way or place of whatever nature, when any part of it is open to the use of the public for vehicular travel.

76-10-501. Definitions.
As used in this part:
(16) "Readily accessible for immediate use" means that a firearm or other dangerous weapon is carried on the person or within such close proximity and in such a manner that it can be retrieved and used as readily as if carried on the person.
(18) "Securely encased" means not readily accessible for immediate use, such as held in a gun rack, or in a closed case or container, whether or not locked, or in a trunk or other storage area of a motor vehicle, not including a glove box or console box.

76-10-505. Carrying loaded firearm in vehicle or on street.
(1) Unless otherwise authorized by law, a person may not carry a loaded firearm:
(a) in or on a vehicle, unless:
(i) the vehicle is in the person's lawful possession; or
(ii) the person is carrying the loaded firearm in a vehicle with the consent of the person lawfully in possession of the vehicle;
(b) on a public street; or
(c) in a posted prohibited area.
(2) Subsection (1)(a) does not apply to a minor under 18 years of age, since a minor under 18 years of age may not carry a loaded firearm in or on a vehicle.
(3) Notwithstanding Subsection (1)(a)(i) and (ii), a person may not possess a loaded rifle, shotgun, or muzzle-loading rifle in a vehicle.
(4) A violation of this section is a class B misdemeanor.

76-10-523. Persons exempt from weapons laws.
(1) Except for Sections 76-10-50676-10-508, and 76-10-508.1, this part and Title 53, Chapter 5, Part 7, Concealed Firearm Act, do not apply to any of the following:
(a) a United States marshal;
(b) a federal official required to carry a firearm;
(c) a peace officer of this or any other jurisdiction;
(d) a law enforcement official as defined and qualified under Section 53-5-711;
(e) a judge as defined and qualified under Section 53-5-711;
(f) a court commissioner as defined and qualified under Section 53-5-711; or
(g) a common carrier while engaged in the regular and ordinary transport of firearms as merchandise.
(2) Notwithstanding Subsection (1), the provisions of Section 76-10-528 apply to any individual listed in Subsection (1) who is not employed by a state or federal agency or political subdivision that has adopted a policy or rule regarding the use of dangerous weapons.
(3) Subsections 76-10-504(1) and (2), and Section 76-10-505 do not apply to:
(a) an individual to whom a permit to carry a concealed firearm has been issued:
(i) pursuant to Section 53-5-704; or
(ii) by another state or county; or
(b) a person who is issued a protective order under Subsection 78B-7-106(1)(b) or 78B-7-404(1)(b), unless the person is a restricted person as described in Subsection 76-10-503(1), for a period of 120 days after the day on which the person is issued the protective order.
(4) Except for Sections 76-10-50376-10-50676-10-508, and 76-10-508.1, this part and Title 53, Chapter 5, Part 7, Concealed Firearm Act, do not apply to a nonresident traveling in or though the state, provided that any firearm is:
(a) unloaded; and
(b) securely encased as defined in Section 76-10-501.

FEDERAL LAWS

18 USC § 926A. Interstate Transportation of Firearms
Notwithstanding any other provision of any law or any rule or regulation of a State or any political subdivision thereof, any person who is not otherwise prohibited by this chapter from transporting, shipping, or receiving a firearm shall be entitled to transport a firearm for any lawful purpose from any place where he may lawfully possess and carry such firearm to any other place where he may lawfully possess and carry such firearm if, during such transportation the firearm is unloaded, and neither the firearm nor any ammunition being transported is readily accessible or is directly accessible from the passenger compartment of such transporting vehicle: Provided, That in the case of a vehicle without a compartment separate from the driver's compartment the firearm or ammunition shall be contained in a locked container other than the glove compartment or console.
There is currently an amendment named H. R. 358 that was introduced to Congress on January 6th, 2017 to make this law better for travelers. See it here.

It is ILLEGAL to carry, have access to, or have a firearm in your vehicle on certain properties which include parking lots
See Parking Lot Laws