We've been having a problem with the drcc (dynamic radar cruse control) on our 2011 Prius. The drcc locks up (freezes) -locking onto a phantom vehicle ahead and activating the beeper/applying the brakes -' even when there is no vehicle is ahead! this creates a safety hazard as the car suddenly decelerates at interstate speeds with no warning to vehicles behind. We have had 7 occurrences of this problem. A search of the htsa database yields two similar reports (10448415 & 10421208). In all cases this has occurred on clear sunny days as the car was passing a semi tractor-trailer on an interstate highway when our car was passing the semi's cab. In most cases, there was no vehicle in front (car in constant speed mode) - it just suddenly acted like there was one! whenever this occurs, the car keeps braking and the only immediate solution is to turn disengage the cruse control. When the temperature is high (above 90 degrees) when we re-engage the drcc, the unit is still locked up! what cleared the locked up condition was turning off the car. On one occasion, when the ambient temperature had dropped below 90 degrees the drcc cleared itself. I have a log of all 7 occurrences of this problem. I have contacted Toyota about this problem (case #S [xxx] & [xxx]) and they had me take the car to my local Toyota dealer who used their techscan tool to download any system generated error codes - they found none. (of course, if the software design of the system does not recognize that malfunction has occurred, no error code will be generated.) the dealer also took the car out on a couple of (short) test drives, but the system performed normally. *note: A possible problem here is that, when hot, the radar unit is picking up signals from the truck's satellite position reporting system. There are several electronic manufacturing faults that could lead to this. Information redacted pursuant to the Freedom Of Information Act (FOIA), 5 U.S.C. 552(B)(6).

- Fairport, NY, USA

The contact owns a 2011 Toyota Prius. The contact stated that the vehicle accelerated unexpectedly after the brakes and start button were depressed intermittently. The vehicle was taken to the dealer. The manufacturer was not contacted. The vehicle was not repaired. The failure and current mileages were 28,600.

- Palm Dale, CA, USA

Drove over a speed hump, and as the front tires hit the level pavement, the car lurched forward. Responded to brakes just fine. Happened again a week later as both front tires simultaneously went over an edge dropoff at a construction area.

- Arlington, VA, USA

The car suddenly decelerated from 65 miles an hour to about 5 miles per hour while driving on a highway at night with the shoulder blocked due to road construction. This same problem was present in the 2004 Prius and was apparently the subject of NHTSA investigation. There was a recall issued on November 30, 2010 for 650,000 units of the 2010 Prius. Toyota has not been able to determine the cause of this problem in the earlier models and admits it doesn't know the cause of the problem in the 2011 Prius. This defect presents a great danger, especially when driving in heavy traffic at highway speeds.

- Victoria, TX, USA

The contact owns a 2011 Toyota Prius. While driving at any speed, the contact stated that the vehicle would suddenly accelerate forward while pressure was being applied to the brakes. The dealer advised the contact to bring the vehicle in for diagnostic testing. The manufacturer took a complaint on the contacts behalf but provided no further assistance. The current mileage was 2,000 and the failure mileage was 900.

- Long Island, NY, USA