What does the 'rollover threshold' of a trailer refer to?

The rollover threshold is the level of lateral acceleration (cornering force) at which a trailer will tip over. It is expressed in terms of gravity (g). A typical empty box trailer has a rollover threshold around 0.5g; a typical loaded box trailer around 0.4g; a typical loaded tanker around 0.35g; and a tanker with partial liquid load may be as low as 0.25g. A passenger car has a rollover threshold around 1.0g — well above the limits of road adhesion, so cars rarely roll. Trucks have thresholds within or below the limits of grip, which is why curves and lane changes that pose no rollover risk to cars can roll heavy vehicles. Drivers reduce rollover risk by reducing speed and avoiding sudden steering.