A commercial motor vehicle stopped on the road is a hazard the rules treat as urgent, and § 392.22 puts two duties on separate clocks. Whenever a vehicle is stopped upon the traveled portion of a highway or its shoulder for any cause other than a necessary traffic stop, the driver shall IMMEDIATELY activate the hazard warning signal flashers, and keep them flashing until the warning devices are placed. Then, as soon as possible but in any event within 10 MINUTES, the driver places the warning devices § 393.95 requires: one on the traffic side, 4 paces (about 10 feet) from the vehicle toward approaching traffic; one at 40 paces (about 100 feet) toward approaching traffic in the center of the lane or shoulder the vehicle occupies; and one at 40 paces in the direction AWAY from approaching traffic. The flashers go back on while the devices are picked up for storage before the vehicle moves.

The general placement is the default, and the section then reshapes it for the situations where the default would not work. On a divided or one-way highway the devices go at 200 feet and 100 feet toward approaching traffic, plus one at the traffic side within 10 feet of the rear. Where the vehicle is stopped within 500 feet of a curve, the crest of a hill or another obstruction to view, the device in the direction of that obstruction goes out to between 100 and 500 feet, far enough to give ample warning. In a business or residential district of a municipality the devices are not required at all — except during the time lighted lamps are required and street lighting is insufficient to make the vehicle clearly discernible at 500 feet. A driver equipped only with fusees places a lighted one at each prescribed location and keeps at least one lit there for as long as the vehicle is stopped, extinguishing and removing each before moving off. And where gasoline or another flammable or combustible liquid or gas seeps or leaks from the vehicle, no flame-producing warning signal may be lit or placed except at a distance that assures prevention of a fire or explosion.

That last provision is the bridge into the chapter’s final group, which is about ignition rather than warning. § 392.50 forbids a driver or any employee of a motor carrier to fuel a commercial motor vehicle with the engine running, except where running the engine is necessary to fuel it; to smoke or expose any open flame in the vicinity of a vehicle being fueled; to fuel unless the nozzle of the fuel hose is continuously in contact with the intake pipe of the fuel tank; or, insofar as practicable, to permit any other person to engage in activities likely to result in fire or explosion. § 392.66 addresses the invisible version of the same danger: no person shall dispatch or drive any commercial motor vehicle, or permit passengers on it, when any of three conditions is KNOWN to exist — an occupant has been affected by carbon monoxide, carbon monoxide has been detected in the interior, or a mechanical condition is discovered that would be likely to produce a carbon monoxide hazard to the occupants — until those conditions have been remedied or repaired. § 392.67 is narrower than its title suggests and worth reading for that: no open flame heater USED IN THE LOADING OR UNLOADING of the commodity transported shall be in operation while the vehicle is in motion. Between them these three rules cover the fuel going in, the exhaust getting out, and one specific flame that must be out before the wheels turn — and the inspection duties that keep a vehicle away from all three are Chapter 11’s subject.