It is every family's worst nightmare: a traffic accident that takes the life of a loved one, often through no fault of their own. Such incidents are usually an agonising, private tragedy for those involved. The allegations in the case of 19-year-old Harry Dunn, however, are a matter of transatlantic diplomacy and threaten to become an embarrassment to the British Prime Minister Boris Johnson.

They are also a reminder that diplomatic immunity is often used as a shield in ways that were never intended. Johnson, who once criticised the absurdity of the protections offered, can't let his voice be muffled this time by his need to keep the Americans onside after Brexit.

On August 27, Dunn's motorcycle collided head-on with a Volvo outside a US intelligence base about 120 kilometres north-west of London; he suffered multiple injuries and was later pronounced dead. Dunn's devastated family say they were told by police that they believe the Volvo driver was travelling on the wrong side of the road.

The driver of the vehicle, named as 42-year-old Anne Sacoolas, is the wife of a US diplomat who may have only been in the country for a short period. Police reported that she was co-operative initially and had no plans to leave the country. But after Dunn's death, Sacoolas claimed immunity and returned to the US with her family.