Toyota Motor Sales U.S.A. Inc. says it is probing complaints from ’10 Toyota Prius drivers who claim their vehicles’ cruise-control systems failed to disengage, or were slow in doing so.

Of the 160 ’10 Prius complaints filed as of March 17 in the “Vehicle Speed Control” category of the National Highway Traffic Safety Admin.’s Safercar.gov database, 11% cite cruise-control issues, Ward’s finds.

More than half of those gripes cite difficulty getting cruise-control to turn off as it should when the vehicle’s brake pedal was depressed.

The observations arise as incidents of sudden unintended acceleration and momentary brake loss are linked to Toyota vehicles.

Last month, Toyota recalled 133,000 ’10 Prius models in the U.S. due to a momentary loss of braking power, occurring when the car travels over rough surfaces. Some complaints in the NHTSA database that cite a cruise-control issue also refer to this problem.

“Cruise control is very difficult to stop with brake. This is consistent,” reads one ’10 Prius complaint in NHTSA’s database. “I have stopped using the cruise control because it has almost caused me to rear-end another vehicle on several occasions.”

Says another complaint: “A driver expects that the cruise control will automatically cancel while applying the brakes, but this does not happen under light braking.”

Toyota insists a tap on the brake should be enough to pull the Prius out of cruise-control.

The auto maker believes some of the ’10 Prius cruise-control complaints are the result of confusion over the system’s function, specifically the adaptive cruise-control system dubbed Dynamic Radar Cruise Control, optional in the top-end Prius V.

Introduced in the ’04 Lexus LS 430 and now available on all Lexus models except the SC convertible, the DRCC uses front-mounted radar sensors to detect a vehicle traveling ahead and paces the trailing car’s speed accordingly.

When the lead vehicle slows down, the trailing vehicle slows down. When the lead vehicle speeds up, the trailing vehicle speeds up.

Drivers using that mode can choose between long, medium and short trailing distances, selected by a steering-wheel-mounted button.

Priuses equipped with DRCC also have a conventional cruise-control mode. However, vehicle-to-vehicle with a long-distance setting is the default mode when a driver activates cruise control in the ’10 Prius.

In vehicle-to-vehicle mode, speeds are set either by modulating the gas pedal and tapping down on the stalk at the desired moment, or by manipulating the stalk up or down then releasing.

Tapping the stalk upward boosts speed in 1-mph (0.6-km/h) increments. But if the stalk is moved up and held, speed increases at 5-mph (8-km/h) increments, a fact not detailed in the DRCC section of the ’10 Prius’ owner’s manual.

“That’s where people are getting confused, I think,” Toyota spokesman Dave Lee tells Ward’s. Lee says the rate at which drivers can reach a desired speed using the stalk is much faster than if they accelerated via the gas pedal, taking about one second to advance every 5-mph increment.

“If you’re going 60 (97 km/h) and want to accelerate to 80 (129 km/h), holding that stalk for about four seconds will get it done,” Lee says. “Trying to do it the traditional way in (vehicle-to-vehicle) radar cruise, it will take a lot longer than four seconds to get to 80.”

Research shows there is a learning curve associated with adaptive cruise control (ACC). “ACC owners tend to be unaware of manufacturer’s warnings and limitations of their system,” says a 2008 survey by NHTSA and the American Automobile Assn.

“They tend to overestimate their system’s effectiveness for preventing collisions in situations where ACC does not perform well for this purpose, such as encountering a stopped vehicle in the lane ahead.”

Owners’ manuals typically state both laser- and radar-based ACC can have difficulties detecting forward vehicles on curves.

Some systems also are subject to malfunction when their sensors are damaged or obscured by dirt. In addition, ACC does not usually operate at speeds below 20-30 mph (32-48 km/h).

Two NHTSA/AAA survey respondents reported colliding with stationary objects when their ACC was activated.

But among the most commonly cited ACC complaint were “unsafe/uncomfortable reductions or increases in speed.”

The survey concludes owners’ manuals are not “an effective strategy” to communicate ACC limitations and recommends “refresher courses” on ACC limitations six-12 months after a vehicle purchase.

It is unclear from NHTSA’s database how many of the ’10 Priuses cited were equipped with DRCC and how many had conventional cruise control.

Toyota says the Prius V makes up about 8% of total ’10 Prius sales.

cschweinsberg@wardsauto.com