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Whether you're a hobbyist or enthusiast who likes making car modifications or maybe just someone who drove down the highway and did a double-take at seeing a vehicle seemingly floating on glowing neon lights next to you, you might be wondering about the legality of "underglow" or neon lights under vehicles.
Most states have pretty specific traffic laws or vehicle codes governing, to varying degrees, vehicle specifications and equipment (including lighting). These laws can get pretty specific and may vary in enforcement and interpretation all the way to the local level. When a vehicle gets modified, an owner runs the risk of getting a traffic ticket if the modification isn't within code. As far as lighting goes, neon lights or underglow lighting on private vehicles are generally not permitted with a few exceptions.
Some jurisdictions make specific allowances or prohibitions on particular colors of lights on a vehicle, as well as for specific individuals or vehicles that may have added lighting. Kansas, for example, has a law stating:
"Any motor vehicle may be equipped with neon ground effect lighting, except that such lighting shall not flash, be any shade of red nor shall any portion of the neon tubes be visible. 'Neon ground effect lighting' means neon tubes placed underneath the motor vehicle for the purpose of illuminating the ground below the motor vehicle creating a halo light effect." Kansas Statutes, Chapter 8, Article 17 section 8-1723(f).
The bottom line is that anyone looking to modify their vehicle, particularly with regards to the lighting department, would be wise to get in touch with their local DMV and/or highway patrol to find out if the modifications are allowed. If you simply wish to err on the side of caution, the vast majority of jurisdictions do not permit neon or underglow (particularly blue and/or red) on vehicles. A vehicle with an unlawful modification can receive a traffic ticket for a mechanical violation. In some states, citations for certain unlawful vehicle modifications are considered "fix-it" tickets or "correctable violations" where, after the problem is fixed, the car owner must get the signature of an authorized person and mail the proof of correction rather than taking it to court.
If the court accepts the correction, the case will be dismissed. On the other hand, a failure to fix the modification within a short time after receiving a "fix-it" ticket will lead to a fine for the unlawful vehicle modification, and the issuance of another ticket which will go on the driver's record. Below are links to some more information on the subject of vehicle modifications, as well as to links to the specific applicable statutes for all states. Please note that laws at the local may also supplement state law.