The UK and the US have agreed to amend an “anomaly” that allowed Harry Dunn death suspect Anne Sacoolas to claim diplomatic immunity.
Mrs Sacoolas – the wife of a diplomat at RAF Croughton, Northamptonshire – was able to leave the UK thanks to the “secret agreement”.
She has been accused of killing the 19-year-old motorcyclist in a crash near the base last August.
Mr Dunn’s mother said the change was a “huge step forward”.
Charlotte Charles told the Press Association (PA) the family’s campaign for Mrs Sacoolas to face criminal proceedings in the UK would continue.
Under the amended rules, relatives of US staff at the air base can face prosecution where they may previously have been immune.
Northamptonshire Police said it understood the changes would not be retrospective but welcomed the move.
Foreign Secretary Dominic Raab said the new arrangements had “closed the anomaly that led to the denial of justice in the heartbreaking case of Harry Dunn”.
He said he appreciated the changes “won’t bring Harry back” but hoped they may “bring some small measure of comfort” to his family.
The US State Department said the amendment was a “reflection of our especially close relationship” with the UK.
Northamptonshire Police said it would continue working with British prosecutors to ensure Mrs Sacoolas was returned from the US to face court proceedings.
Mrs Sacoolas, 42, was charged with causing death by dangerous driving in December.
A Home Office extradition request was refused by US secretary of state Mike Pompeo in January, and American officials said the decision was final.
It is believed Mrs Sacoolas was driving on the wrong side of the road when Mr Dunn was killed.
There have been reports of other vehicles spotted driving on the wrong side of the road near the base, including one which was shown in a YouTube video in February.