A reported runaway Toyota Prius in California has had US safety investigators involved in determining the cause of a terrifying ride.

The current high-profile case involves a San Diego driver, James Sikes, 61, who has claimed he was driving his 2008 Prius on a busy freeway when the car started to accelerate of its own accord.

The California Highway Patrol became involved when the car started racing at around 140 kilometres per hour.

After a panicked 911 call, Highway Patrol officers used a loudspeaker to talk Sikes through an emergency stop by using his handbrake and turning off the engine.

The National Highway Traffic Safety Administration has started to take evidence of the incident and are currently inspecting the vehicle.

Toyota has also sent its own technical specialists to San Diego to assist NHTS officers.