The short answer
The O code sets no distance. It asks only that a leak from one package could not reach the other, which states a result rather than a measurement you can check.
The four-foot figure comes from another section. It governs one narrow pairing of hazards, and the agency offered it for the table only as an opinion rather than a floor.
Two drums stand side by side on a trailer floor. One holds a flammable liquid and the other an oxidizer, and the table marks that pair O.
So they may ride together, provided something separates them. Then comes the question the table cannot answer, which is how far apart they have to sit.
The rule gives no figure, and asks instead that a leak from one package could not reach the other. That is an outcome, not a dimension you can check with a tape.
Meanwhile a figure of four feet goes around, and some training material states it as the rule. It is real federal text, but it belongs to another section.
Two sections away the rule does set distances, four feet in one place and nine in another. At sea the same idea carries four separate figures.
This reference covers the codes and what each commands, the pairings barred outside the table, and where the distances live.
The Codes, and Three Pairings Outside the Table
The segregation table is a grid, with hazard classes down the side and across the top. The cell where two of them meet holds one of four codes.
| Code | What it commands |
|---|---|
| X | Not loaded, carried or stored together in one vehicle or site |
| O | Not together unless separated so leakage would not commingle |
| Blank | No restrictions apply |
| * | Explosives run on a second table |
| A | Ammonium nitrate may go with Division 1.1 or 1.5 |
Read the X carefully, because it reaches past the truck: the ban covers a storage site during transport, so a cross-dock falls under it too.
Then there is the part before the grid, where paragraph (c) bars three combinations outright. Cyanides may not travel with acids where the mixture would make hydrogen cyanide, and Division 4.2 goods may not travel with Class 8 liquids. The most toxic inhalation group may not travel with flammable liquids, corrosive liquids, or four of the flammable and oxidizing divisions.
That third ban is the broadest in the section, and it sits in prose rather than a cell. Waste shipments get a narrow way out elsewhere.
The Code With No Number
Here is the whole of it. Goods marked O may not travel together unless separated in a manner that, if packages leaked in normal transport, commingling would not occur.
That sentence names no distance, no barrier type and no method, and it leaves the means to you. Which is odd for a rule carrying penalties.
A waste firm asked the agency about it in 2008, wanting a minimum distance, and the reply is worth quoting twice. First: “There is no minimum distance specified.” Then, in the next breath: “a distance of four (4) feet in all directions would be considered acceptable.”
An opinion offered, not a floor set. The same letter then closes a door most shippers assume is open, because “the mere integrity of a packaging may not be used as a physical barrier.” So the drop and leakproofness results behind UN certified packaging count for nothing here.
Absorbent fares better, but only on condition. A bag of vermiculite between a Class 3 drum and a Division 5.1 drum works, the letter says, only if it does prevent commingling.
One pairing gets a flat geometry rule even so. The rule bars Class 8 liquids from riding above or next to Class 4 or Class 5 materials, whatever gap you leave. A shipper may still combine truckload amounts where the mixture is safe.
Where the Rule Does Set a Distance
Two sections earlier in the same part, distances appear that are specific and numbered, and neither of them governs the table.
| Section | Distance | What it covers |
|---|---|---|
| 177.838(i) | 1.2 m (4 ft) | Division 4.2 self-heating goods near Class 8 |
| 177.841(f) | 2.74 m (9 ft) | Toxic inhalation group with Class 3, 4, 5 and 8 |
| 177.841(e) | Flat ban | Poison-labelled packages with food or feed |
| 177.848(e)(3) | None | Every pairing the table marks O |
So the four-foot figure is genuine, and it has drifted. In its own section it attaches to one pairing of self-heating solids and corrosives, and in circulation it has become the general meaning of the O code.
A study guide for the hazmat driving test states it as a required minimum, adding an approved floor-to-ceiling barrier as the other option. Neither phrase appears in the section at all.
The foodstuffs rule is stricter than anything in the table. A poison-labelled package may not share a vehicle with food, and only two exits exist. Overpack the poison in a metal drum, or load each into its own closed unit load device.
At Sea, the Same Idea Carries Figures
Put those two packages on a ship and the vagueness disappears, because the vessel section defines four degrees of segregation, and three carry a figure.
| Degree | On deck |
|---|---|
| Away from | 3 m (10 ft) horizontal |
| Separated from | 6 m (20 ft) |
| Separated by a complete compartment | 12 m (39 ft) |
| Separated longitudinally | 24 m (79 ft) |
Containers get their own math, since a box space counts as at least 6 m fore and aft.
Which makes the gap hard to explain away, because the highway rule sends a vehicle carried by vessel into those vessel rules. So one trailer crosses from a rule with no number into a rule with four, and its contents never change.
The Class You Looked Up May Not Govern
A package can carry two hazards. When the hazmat table calls for a second label, the rule for that hazard applies wherever it is stricter, so the cell you read may be the wrong one.
An exception runs the other way, since goods of the same class may share space whatever their second hazard, provided they cannot react to cause heat, fire or dangerous gas. Hazard class does not settle the package either, which is why lithium battery packaging answers to rules inside the box, not this table.
Explosives work differently again, because groups A through S replace the classes. A shipment mixing divisions within one group travels as the lower division. So 1.1 governs a load that also holds 1.2, and 1.5D carried with 1.2D in one box ships as 1.1D.
And the instructions are not finished. In December 2024 a trooper asked how to segregate an organic peroxide whose explosive subsidiary has no division listed. The agency advised treating it as Division 1.1, the most restrictive reading, and said it may clarify the instructions later.
Segregation Is Not Securement
The hardware overlaps, which is why the two rules get confused. Barriers, dividers, inert freight and empty space are the agency’s list for separation, and dunnage does that job as readily as it blocks a pallet.
But the questions differ, because the cargo securement rule asks whether the load stays put and answers in force. Segregation asks whether two leaks could meet, and answers with no figure at all.
So a specification should keep them apart on the page. The separation standard is an outcome, so the defense of a load is the thinking behind it, which makes a photograph of the load worth keeping.
Common Questions
How far apart must packages marked O be?
The rule names no distance, and asks only that a leak from one package could not commingle with the other. The agency has said in writing that no minimum is specified, while offering four feet as acceptable.
Does a UN-certified drum count as separation?
No. The agency has said the mere integrity of a packaging may not serve as a physical barrier. The gap must come from something between the packages, such as a divider, inert freight or space.
Where does the four-foot rule come from?
From a different section, which sets four feet for Division 4.2 self-heating goods near Class 8. It is not the meaning of the O code, whose own section names no distance.
Can poisons travel with food?
Not normally, since a package bearing a poison label may not share a vehicle with food or feed. Two exits exist: overpack in a metal drum, or load each into its own closed unit load device.
How to weigh this page. The codes, bans and distances come from the current text of the federal highway and vessel sections, read directly. The quoted lines on distance and packaging come from the agency’s own letters, dated 2008 and 2024.
This is guidance for writing a specification, not safety advice, and it does not replace the regulations. Two things stay out on purpose. The grid is a wide matrix that belongs on the agency’s page, and the explosives table is given in brief.
Key takeaways
- The O code requires only that leakage could not commingle, and sets no distance.
- The four-foot figure belongs to a separate section, and was offered for the table only as an opinion.
- A packaging cannot be its own barrier, so certification results do not help a separation case.
- At sea the same idea is measured, the four degrees running 3, 6, 12 and 24 m.
- A second label can move a load to a stricter row, and explosives take the lower division.