James Sikes, a Prius driver near San Diego, went to pass another vehicle
Monday when, he says, he noticed the car continuing to accelerate.
After he passed 90 mph, he called 911.

Originally, it was reported that a California Highway Patrol officer
used his car as a brake for the Prius, but that turns out not to be the
case. The rest of the story from the California Highway Patrol:

“The California Highway Patrol immediately went through a series of
actions with negative results.

“CHP Officers immediately responded to the area were able to locate the
vehicle eastbound I-8 east of Kitchen Creek Rd.  A CHP Officer was able
to pull along side and instruct Mr. Sikes via the PA system. With the
Officer's assistance, he was able to slow the Prius to approximately 50
m.p.h. The Officer then placed his patrol vehicle in front of the Prius
as it came to a stop in the number #1 lane of e/b I-8.  The Officer then
placed the patrol vehicle against the front bumper of Prius.   

“The vehicles did not touch until after they came to a stop.”

Toyota executives said a technical specialist his been dispatched to
look at the car.

Runaway
Toyota Prius Stopped By California Patrol Car (Jalopnik)