Most of the hazmat rules you study for the CDL test are about paperwork and placards. A small group of rules is about something more immediate: the moments when a vehicle full of hazardous materials is most likely to meet heat or a spark. Fueling is one. Smoking is another. A tire that overheats and catches fire is the third. Each has its own short rule in 49 CFR Part 397, the part of the federal regulations that covers driving and parking vehicles that carry hazardous materials. The rules are brief, they are easy to memorize, and they show up on the hazmat knowledge test. This guide explains each one, what it actually says, the common myths around it, and how to remember it.
This is study guidance, not regulatory advice. The binding text is the regulation and your state’s adoption of it, and your official state CDL manual is the authority for the test.
Who these rules apply to
Part 397 applies to motor carriers transporting hazardous materials in a vehicle that must be marked or placarded, and to their drivers and other employees involved in that transport. In other words, these are rules for placarded loads. If you are not sure when a load needs placards, start with our guide to when hazmat placards are required.
Part 397 also says that a vehicle containing hazardous materials must be driven and parked in compliance with the laws of the place where it operates, unless a federal rule is stricter. That matters because many states and cities add their own rules on top of the federal ones, especially about where hazmat vehicles may park and travel.
The fueling rule
The federal fueling rule, 49 CFR 397.15, has two parts. When a motor vehicle containing hazardous materials is being fueled:
- Its engine must not be operating.
- A person must be in control of the fueling process at the point where the fuel tank is filled.
That is the entire rule. It is short because the danger it addresses is simple. A running engine is an ignition source next to fuel vapour. And an unattended nozzle can overflow, spill fuel around a vehicle carrying hazardous materials and spread a fire before anyone notices.
What “in control” means in practice
The person does not have to be the driver, but someone must be at the nozzle, controlling the flow, for the entire time fuel is going in. That rules out the common habit of setting the nozzle latch and walking into the truck stop. If something goes wrong, the person at the nozzle stops the flow immediately.
Good practice at the fuel island goes beyond the rule:
- Shut the engine off before you open the fuel cap, not after you start fueling.
- Stay at the nozzle for the whole fill.
- Do not overfill. Leave room for expansion, especially in hot weather.
- Clean up small spills before restarting.
- Know where your fire extinguisher is. A power unit carrying a placarded load must carry an extinguisher with a UL rating of 10 B:C or more, under 49 CFR 393.95.
A test trap about fueling
Test questions often offer an answer like “you may leave the engine running if the weather is cold” or “you may leave the nozzle unattended if it has an automatic shutoff”. Both are wrong. The rule has no weather exception and no automatic-shutoff exception. Engine off, person at the nozzle.
The smoking rule
The smoking rule, 49 CFR 397.13, says no person may smoke or carry a lighted cigarette, cigar or pipe on or within 25 feet of:
- A motor vehicle that contains Class 1 explosives, Class 5 oxidizers and organic peroxides, or flammable materials classified as Division 2.1 (flammable gas), Class 3 (flammable liquid), or Divisions 4.1 and 4.2 (flammable solids and spontaneously combustible materials).
- An empty tank motor vehicle that has been used to carry Class 3 flammable materials or Division 2.1 flammable gas, and that was required to be marked or placarded when so used.
How to remember the classes
The list is easier to remember once you notice what the classes have in common: they either burn, explode, or feed a fire.
| Class | Why smoking is banned |
|---|---|
| Class 1 explosives | A spark can detonate |
| Division 2.1 flammable gas | Vapour ignites easily |
| Class 3 flammable liquid | Vapour ignites easily |
| Division 4.1 flammable solid | Ignites from heat or friction |
| Division 4.2 spontaneously combustible | Already prone to self-heating |
| Class 5 oxidizers and organic peroxides | Supply oxygen that makes fires burn hotter |
The oxidizers are the entry people forget. An oxidizer does not have to burn by itself to be dangerous; it makes everything around it burn faster and hotter. Our guide to Class 5 oxidizer and organic peroxide placards explains why.
Notice too that the rule covers the empty tanker. A tank that carried gasoline and has not been cleaned still contains flammable vapour, and that vapour is often more dangerous than a full tank, because there is more space for a flammable mixture of vapour and air. Our guide to residue and empty tank placarding covers the empty tank rules.
”On or within 25 feet”
The distance is 25 feet in every direction, and “on” the vehicle includes the cab. A driver hauling a Class 3 load may not smoke in the cab. People standing near the vehicle, including helpers, other drivers and bystanders at a loading dock, are covered too. A driver who sees someone lighting up near the truck should ask them to move.
The same 25 foot distance appears in the loading and unloading rules, covered in our guide to hazmat loading and unloading safety rules, so it is worth learning once and applying in both places.
The tire rule
The tire rule, 49 CFR 397.17, says a driver must examine each tire on the vehicle at the beginning of each trip and each time the vehicle is parked.
If a tire is found to be flat, leaking or improperly inflated, the driver must have it repaired, replaced or properly inflated before the vehicle is driven, although the vehicle may be driven to the nearest safe place to do that work.
If a tire is found to be overheated, the driver must immediately have it removed and placed at a safe distance from the vehicle, and must not drive the vehicle until the cause of the overheating is corrected.
Why tires get their own rule
An underinflated tire flexes more, builds heat, and can eventually catch fire. Tire fires are hard to put out, they can reignite after they appear to be out, and on a vehicle carrying flammable or explosive materials they can spread to the cargo. A dual tire that has gone flat is especially dangerous, because the good tire beside it carries the extra load and heats up while the flat one rubs.
That is why the rule asks for a check every time the vehicle stops, not just once a day. A walk-around at each stop catches the slow leak before it becomes a fire.
A myth about the tire rule
Older study materials and some practice tests describe a rule requiring tire checks every two hours or every 100 miles. The current federal text of 397.17 does not say that: it requires an examination at the start of each trip and each time the vehicle is parked. Some state manuals still carry the older wording, and your state test is written from your state manual, so check what your manual says and answer the test the way your manual teaches it. On the road, checking at every stop meets the federal rule and is simply good practice.
How to check a tire
A proper tire check is quick. Use a tire gauge for pressure, because a thump or a kick will not reliably tell you that a tire is underinflated. Feel or look for heat, especially on duals. Look for cuts, bulges, exposed cords and objects stuck in the tread. Check that dual tires are not touching each other. If anything looks wrong, deal with it before driving on.
Open fires
A closely related rule, 49 CFR 397.11, covers fires the driver did not start. A vehicle containing hazardous materials must not be driven near an open fire unless the driver has first made sure the vehicle can safely pass the fire without stopping, and it must not be parked within 300 feet of an open fire. That includes burning fields, roadside brush fires and fires at a nearby site. If you come upon a fire across your route, the right move is often to stop well back, out of the 300 foot zone, and wait, rather than trying to drive through.
Why these rules are on the test
These rules are popular test material because they turn on precise words and numbers that can be checked:
- Fueling: engine off, and a person at the nozzle.
- Smoking: 25 feet, and the listed classes, including empty flammable tanks.
- Tires: at the start of each trip and each time you park, with overheated tires removed and placed away from the vehicle.
- Fires: do not park within 300 feet of an open fire.
A good way to study them is to say each one out loud in a single sentence, then test yourself with practice questions. Our guide to why people fail the hazmat test explains why numbers like 25 and 300 are where most points are lost, and hazmat flashcards vs practice tests shows how to combine the two study methods.
Common mistakes
Starting the fill, then walking away. The nozzle latch is not a person. Stay at the nozzle.
Smoking in the cab with a flammable load. “On the vehicle” includes the cab.
Forgetting oxidizers and empty tanks. Both are on the smoking list, and both catch out test takers.
Kicking tires instead of gauging them. A kick does not measure pressure. Use a gauge and check duals for heat.
How the rules fit together
These rules share one idea: keep heat and sparks away from hazardous materials. The railroad crossing rule in our guide to hazmat railroad crossing rules and the attendance rules in hazmat parking and attendance come from the same thinking about risk. Learn them as one group and they are easier to remember than as separate facts.
Frequently asked questions
Can you leave the engine running while fueling a hazmat truck?
No. Under 49 CFR 397.15, when a vehicle containing hazardous materials is being fueled, the engine must not be operating, and a person must be in control of the fueling process at the point where the tank is filled.
How far away must you be to smoke near a hazmat vehicle?
More than 25 feet. Nobody may smoke or carry a lighted cigarette, cigar or pipe on or within 25 feet of a vehicle carrying Class 1, Class 5, Division 2.1, Class 3, or Division 4.1 and 4.2 materials, or an empty tank that carried Class 3 or Division 2.1 and was placarded.
How often must you check tires on a hazmat vehicle?
At the beginning of each trip and each time the vehicle is parked, under 49 CFR 397.17. Some state manuals still mention a check every two hours or 100 miles, so study the rule as your state manual presents it.
What do you do with an overheated tire on a hazmat truck?
Remove it immediately and place it at a safe distance from the vehicle, and do not drive until the cause of the overheating is corrected.
What is the best way to memorize hazmat fueling and smoking rules?
Reduce each rule to one sentence, memorize the numbers 25 and 300, and learn which classes the smoking rule covers by recognizing their placards. An app such as CDL Placards helps with fast placard recognition, so you know at a glance when your load triggers the rule. Your state CDL manual is the authority.
