The off-duty officer was driving to work near the town of Bruchsal-Büchenau on the A5 in the southwestern German state of Baden-Württemberg when he saw the driver of a Renault with French license plates sleeping openly, his head lolling against the window as his companion in the front passenger seat steered the car at a speed of about 110 kilometres (68 miles) per hour, Karlsruhe police said.
The officer - who was not in uniform - followed the car, honking his horn repeatedly as the driver slept.
More than nine kilometres later, after his copilot swerved wildly while changing lanes, the driver awoke and accelerated to about 160 kilometres per hour, according to the officer's report.
Police intercepted the car just north of the city of Karlsruhe.
The passenger, also a French national, was under the influence of heroin and had a blood alcohol content of nearly 0.05 percent, police said. He being investigated on suspicion of driving while drunk, under the influence of drugs and without a driver's license.
The sleeping driver was sober and is not being charged, according to the police.