Radar Detectors Are Unregulated in Montana Cars and Banned in Its Commercial Trucks

Montana regulates vehicle equipment in one place, Title 61, chapter 9, which runs in five parts: general provisions, lighting equipment, brakes, miscellaneous regulations and enforcement. A full read of every section in that chapter in the 2025 edition of the Montana Code Annotated returns no instance of the words radar, detector, jammer, scanner or laser. There is no prohibition, no permit, no confiscation provision and no penalty. The nearest Montana law comes to the subject is 61-9-109(3), MCA, which states in one sentence that "The use of additional parts and accessories on a vehicle not inconsistent with the provisions of this chapter is not prohibited." A detector that neither obstructs the driver's view nor emits light in a way chapter 9 forbids is exactly such an accessory.

The only radar sections in the traffic code point at the police, not the motorist. Section 61-8-702, MCA, provides that the speed of a motor vehicle may be measured by radio microwaves or other electrical device and that the results are accepted as evidence of speed in any court or legal proceeding where speed is at issue. Section 61-8-703 governs a warrantless arrest in a radar case: the arresting officer must be in uniform or display a badge of authority and must either have observed the recording of the speed personally or received a radio message from the officer who did, giving the license number or other sufficient identification and the recorded speed, dispatched immediately after the speed was recorded. Subsection (2) requires that the arrest be made immediately after the observation or radio message and as the result of uninterrupted pursuit. Nothing in either section addresses a device that detects the emission.

Local government cannot quietly fill the gap with a conflicting ordinance. Section 61-9-103(1), MCA, makes the equipment chapter applicable and uniform throughout Montana and in all its political subdivisions and municipalities, and forbids a local authority from enacting or enforcing any ordinance, rule or regulation in conflict with the chapter unless the chapter expressly authorizes it. The same subsection does leave room for local authorities to adopt additional traffic regulations that are not in conflict, so the uniformity clause is a limit on contradiction rather than a total ban on local rulemaking.

Commercial drivers are in a different position, and the rule that binds them is administrative rather than statutory. Section 61-10-154(2), MCA, directs the Department of Transportation to adopt safety standards by rule for for-hire and private motor carriers, for vehicles of 10,001 pounds or more used in interstate commerce, for vehicles of 26,001 pounds or more used in intrastate commerce that are not farm vehicles operating solely in Montana, for vehicles designed or used to carry at least 16 passengers including the driver, for vehicles carrying at least nine passengers for compensation, and for placarded hazardous materials vehicles. The rule adopted under that mandate is ARM 18.8.1502, whose current version took effect on May 9, 2026 under Montana Administrative Register Notice No. 2026-23. Subsection (1) adopts by reference a long list of Federal Motor Carrier Safety Regulations including 49 CFR parts 390 through 399, as updated through March 10, 2026. Part 392 sits inside that range, and 49 CFR 392.71 prohibits driving a commercial motor vehicle equipped with or using a radar detector.

The state modifications in ARM 18.8.1502(2) are worth checking before assuming a federal rule applies unaltered in Montana, because the department did carve several out. The listed modifications touch 49 CFR part 385 (renaming the intrastate safety investigation), part 387 (financial responsibility, with 61-6-103, MCA, setting the intrastate minimum), 390.19 and 390.21 (the MCS-150 report and a state USDOT number followed by the abbreviation MT), part 391 (age and physical qualification under ARM 23.3.505 and 23.3.506 for type 2 commercial licence holders), 395.1(k) and 395.8 (planting and harvesting seasons deemed to run January 1 through December 31, and a six-month daily hours record), and 396.9 (driver presence at a CVSA Level I, II, III or V inspection). Part 392 is not modified, so it applies to Montana carriers as written.

There is no offense to charge a private motorist with. A Montana officer who sees a detector on the windscreen has no equipment provision to cite, and 61-9-511, the general penalty for the equipment chapter, only bites on a violation of that chapter. The one way a detector can attract a citation in a passenger car is indirectly, if the way it is mounted breaches 61-9-405(2)(a), MCA, which prohibits driving with a sign, poster, substance or other nontransparent material on the windshield, side wings or side or rear windows that materially obstructs, obscures or impairs the driver's clear view of the highway.

For commercial drivers the penalty is concrete. Section 61-10-154(7), MCA, provides that a violation of the standards adopted under that section is punishable as provided in 61-9-512, and 61-9-512(2) sets the fine at not less than $25 and not more than $500 for a first offense and not less than $25 and not more than $1,000 for each subsequent offense, with the same penalties applying under 61-9-512(3) to a motor carrier that is a corporation. Enforcement is shared: 61-10-154(4) requires the Department of Transportation to work with the Montana Highway Patrol, and 61-10-154(5) lets the department designate motor carrier services employees as peace officers who may issue citations and make arrests for violations of the adopted standards.