The United Kingdom and the United States agreed to amend an “anomaly” that allowed Harry Dunn’s death suspect, Anne Sacoolas, to claim diplomatic immunity.
Ms Sacoolas, the wife of a diplomat at RAF Croughton, Northamptonshire, was able to leave the UK thanks to the “secret deal”.
She was accused of killing the 19-year-old motorcyclist in an accident near the base last August.
Dunn’s mother said the change was a “big step forward.”
Charlotte Charles told the Press Association (PA) that the family’s campaign for Ms Sacoolas to face criminal prosecution in the UK would continue.
Under the amended rules, relatives of US personnel at the air base can be prosecuted when they have previously been immune.
Northamptonshire Police said they 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 Harry Dunn case.”
He said he appreciated the changes “it won’t bring Harry back,” but he hoped they would “bring some comfort” to his family.
The US State Department said the amendment was “a reflection of our especially close relationship” with the United Kingdom.
Northamptonshire Police said they will continue to work with British prosecutors to ensure that Ms. Sacoolas is returned from the United States to face legal proceedings.
Ms. Sacoolas, 42, was accused of causing death from dangerous driving in December.
US Secretary of State Mike Pompeo rejected an extradition request from the Interior Ministry in January, and US officials said the decision was final.
Ms. Sacoolas is believed to have been driving on the wrong side of the road when Mr. Dunn was killed.
There have been reports of other vehicles seen driving on the wrong side of the road near the base, including one that was shown in a YouTube video in February.