In Utah neon underglow is regulated by Utah Code, Title 41: Motor Vehicles, Chapter 6a: Traffic Code, Part 16: Vehicle Equipment.

Is neon underglow legal in Utah?

Utah laws restrict all non-approved vehicle lighting which does include neon underglow. In Utah, neon underglow is considered illegal. Drivers are especially advised to avoid these restrictions:

• No red or blue lights may be visible from the front of the car
• No flashing or rotating lights are allowed

To be safe (even on private property) we recommend avoiding red and blue colors altogether, and any combination of red, blue and white lights which may imitate an emergency vehicle. Furthermore, we strongly advise using any rotating or flashing lights.

According to our direct contact with Utah Highway Patrol, unless the vehicle is on private property, off-road or being used for display purposes only, all underglow is illegal.

Utah vehicle lighting laws

Below are all relevant excerpts from Utah Vehicle Code that limit, restrict or allow certain aftermarket lights to be installed on vehicles.

41-6a-1618. Sale or use of unapproved lighting equipment or devices prohibited.

41-6a-1618. Sale or use of unapproved lighting equipment or devices prohibited.
(1) Except as provided under Subsection (2), a person may not use, have for sale, sell, or offer for sale for use on or as a part of the equipment of a motor vehicle, trailer, semitrailer, or pole trailer any head lamp, auxiliary fog lamp, rear lamp, signal lamp, required reflector, or any parts of that equipment which tend to change the original design or performance, unless the part or equipment complies with the specifications adopted under Section 41-6a-1601.
(2) The provisions of Subsection (1) do not apply to equipment in actual use prior to July 1, 1979 or to replacement parts of this equipment.
(3) A person may not use on a motor vehicle, trailer, semitrailer, or pole trailer any lamps under this section unless the lamps are mounted, adjusted, and aimed in accordance with this part.
(4) A violation of this section is an infraction.

41-6a-1616.  High intensity beams — Red or blue lights — Flashing lights — Color of rear lights and reflectors.

[…] (2) (a) Except for an authorized emergency vehicle described in Section 41-6a-1601, a school bus described in Section 41-6a-1302, or a simulated emergency vehicle used in accordance with Section 41-6a-1718, a person may not operate or move any vehicle or equipment on a highway with a lamp or device capable of displaying a red light that is visible from directly in front of the center of the vehicle.
(b) Except for a law enforcement vehicle, or a simulated emergency vehicle used in accordance with Section 41-6a-1718, a person may not operate or move any vehicle or equipment on a highway with a lamp or device capable of displaying a blue light that is visible from directly in front of the center of the vehicle.

(3)    A person may not use flashing lights on a vehicle except for:
(a)    taillights of bicycles described in Section 41-6a-1114;
(b)    authorized emergency vehicles described in Section 41-6a-1601;
(c)    turn signals described in Section 41-6a-1604;
(d)    hazard warning lights

(4)    Except for an authorized emergency vehicle described in Section 41-6a-1601, or a media production vehicle used in accordance with Section 41-6a-1718, a person may not use a rotating light on any vehicle.
(5)    A violation of this section is an infraction.