Supplementary Liability Insurance (SLI) is sometimes also known as Additional Liability Insurance (ALI)
It is an insurance policy which typically increases the third party insurance that is proved by your car rental firm up to US$1m.
It is called Supplementary Liability Insurance as it augments rather than replaces the cover provided by hire firms. It is also an unnecessary insurance if your car hire company is already providing you with third party insurance of US$1m or more, as it only tops up your cover to the US$1m mark and, therefore, will not pay out if you are already covered to that level. As Western European car hire companies almost always already provide insurance to around €1m you are very unlikely to require it in this region.
However, in many parts of the rest of the world, cover is only provided to a very low level. In the US, for example, the minimum differs from state to state and some car hire firms only need to provide you with a few thousand pounds worth of cover. Often in the Caribbean you can find that you are only covered up to US$25,000. A state by state guide can be found on our faq page covering Supplementary Liability Insurance Limits for Car Hire in the US.
What is frustrating is that often car hire firms do not tell you to what limit they do cover you - if you are in any doubt do check directly with them.
This content was last reviewed on 29/11/2016