PC Jacob Marshall was found guilty of careless driving earlier this month after his Central Scotland Police Volvo hit a car on the M9 near Falkirk.
PC Marshall, 31, had originally faced a charge of dangerous driving, but was convicted of the lesser offence after a two-day trial.
The crash happened as off-duty taxi driver Fraser Lenny, 49, in a Vauxhall Zafira with an 80-year-old passenger, pulled out to allow other traffic from the M876 to merge on to the M9.
Falkirk Sheriff Court heard that the police car clipped the wing mirror of the other vehicle at 121 mph and narrowly avoided the central crash barrier.
The court saw a video of the police car reaching speeds of 149mph on the way to the spill.
Sheriff William Gallacher said he couldn’t understand why the experienced policeman had not slowed down “even a bit” as he approached the busy junction.
He said the high speed was an important factor in convicting Marshall of careless driving – but not the sole factor.
Sentence was deferred until yesterday.
Defence agent David O’Hagan cited a High Court judgment from 1997, where a fireman was allowed to keep his licence after he crashed his engine into a car while on his way to a blaze.
Mr O’Hagan said: “Like in this  case, it’s a journey that he wouldn’t have taken if it wasn’t for the emergency.”
Sheriff William Gallacher told Marshall that he had found enough reason not to endorse his licence or fine and ban him.
He said: “The circumstances presented in the trial showed that PC Marshall had been driving at a very high speed. I did not conclude that this was driving dangerously. But there was a point when he didn’t act with an appropriate level of care.”