Roll-over mechanisms are installed in the manlids on the top of the tanker, which close the aperture in the case of a rollover. The risk on the top of the tanker is that the expansion and contraction of the gasoline liquids imply that each compartment must be allowed to breathe, otherwise the pressure in the tank would catastrophically shatter it.
There would be an apparent leak channel for the liquid to escape if the tanker rolled over, which might result in a fire or an environmental disaster. This rollover mechanism in the manlids can stop the leakage of liquid which prevents environmental disasters or tanker explosions. Read more