How does the segregation distance between two different IMDG classes change based on the mode of transport (e.g., on deck vs. under deck)?

The segregation distance between different IMDG classes, as defined in the IMDG Code segregation table, often increases when dangerous goods are stowed under deck compared to on deck because under-deck stowage presents a greater risk in the event of an incident, such as a fire or spill. The rationale behind this difference is that incidents occurring under deck are more difficult to control due to limited access, ventilation, and escape routes. Therefore, increased segregation distances are required to minimize the potential for incompatible dangerous goods to interact and exacerbate the incident. For example, certain classes of explosives may require greater separation from flammable liquids when stowed under deck compared to on deck to reduce the risk of a fire spreading to the explosives. The segregation table in the IMDG Code specifies the required segregation distances using terms such as "away from," "separated from," "separated by a complete compartment or hold from," and "separated longitudinally by an intervening complete compartment or hold from." These terms have specific meanings in terms of the physical separation required between the dangerous goods. When stowing dangerous goods under deck, stricter segregation requirements may be applied, requiring greater physical separation or the use of fire-resistant barriers. On-deck stowage generally allows for better ventilation and easier access for firefighting, reducing the need for such stringent segregation in some cases. Therefore, the mode of transport significantly affects the required segregation distances for dangerous goods, with under-deck stowage typically requiring greater separation to mitigate the increased risks.