A Grampian Police chief inspector and a colleague have gone on trial accused of dangerous driving on the way to a conference.
Ch Insp Tom Forrester, 45, and Pc Ashley Forbes, 33, were charged after a collision in November 2008.
The incident happened on the B977 Belhelvie to Dyce road in Aberdeenshire.
The two men deny the charges and went on trial at Aberdeen Sheriff Court on Thursday.
Motorist Carol Dawson told the first day of the trial a blue BMW Mr Forbes was driving left the scene of a collision but returned 15 minutes later.
She said he then breathalysed her, and told her he had been on the way to an emergency and had seen the crash happen.
She said a Fiesta had driven into the back of her Volkswagen Golf when she slowed down to avoid colliding with the BMW, which was coming towards her with flashing lights on the wrong side of the road.
Fiscal depute Mhairi Morrison asked her: "Why did you slow down?"
She replied: "Because I saw a vehicle with blue flashing lights on my side of the road.
"There was other traffic so I figured I had to stop."
Ms Morrison asked: "What did you think when you saw the vehicle?"
She replied: "That I had to pull over and get out of its way or I would collide with it. I was a bit shocked because it wasn't what I expected first thing in the morning.
"I slowed down as much as I could and tried to pull into the side of the road."
Ms Dawson said she saw the Fiesta behind her and thought the driver would notice her braking but it came into the back of her car as she stopped.
Mr Forbes was said to have been driving the head of roads policing and another colleague to Aberdeen Airport in the unmarked BMW.
Mr Forrester and Insp James Wood were due attend a roads policing conference in Birmingham.
Mr Wood, 44, told the court: "The chief inspector made the comment that we shouldn't have come on to that road at that time of the morning.
"Constable Forbes asked 'what would you like me to do?'
"The chief inspector considered his options then instructed Constable Forbes to put on the lights and sirens."
Asked how he felt about activating the sirens, he replied: "I wasn't sure about the position of the terms of making use of the lights at that time.
"But I was aware that the chief inspector was an experienced supervisor and had a long time within policing and was head of the department and if anyone would know if you could then it would be him.
"I still felt uncomfortable but was reassured."
The court also heard evidence from Fiesta driver Ian Milne who said he could do nothing to avoid colliding into the back of Ms Dawson's car.
Both officers face a charge of dangerous driving. Mr Forbes lodged a plea of guilty to a reduced charge of careless driving which was not accepted by the Crown.
The trial continues later this year.