LISTEN: What are private prosecutions and will they go ahead?
Only two have been granted since 1900 but two bids for rare private prosecutions have been submitted in the past two days.
The families of two young students who were killed when the driver blacked out in Glasgow and hit them submitted their Bill of Criminal Letters to the Lord Advocate on Thursday.
Mhairi Convy, 18, and Laura Stewart, 20, were walking in North Hanover Street on December 17 2010 when a Range Rover apparently lost control, mounted the kerb and hit them.
The family of teenager Erin McQuade and her grandparents Jack and Lorraine Sweeney who died in the tragic bin lorry crash in George Square in 2014 asked for permission to privately prosecute driver Harry Clarke just a day earlier.
A Bill of Criminal Letters asks the Lord Advocate for permission to take leave to prosecute. In the Scottish legal system, the Lord Advocate generally conducts all prosecutions except under very special circumstances.
In both cases, neither driver has previously been convicted in relation to the incidents.
Range Rover driver William Payne, who was 50 at the time of the crash, appeared at Glasgow Sheriff Court in November 2012 accused of causing death by driving while uninsured but charges against him were dropped the following November.
A fatal accident inquiry (FAI) was held in 2014 into the deaths of the two women and Sheriff Andrew Normand found five ''reasonable precautions'' could have prevented the accident.
In Harry Clarke's case, charges were ruled out before the FAI into the bin lorry crash last year.
Criminal law expert Professor James Chalmers from University of Glasgow has been talking to our reporter Lizzie Parker about private prosecutions and how likely it is that the cases will go ahead: