Avoid Accelerating Accidentally with the Brake Override System

Break Override System contributes to safer driving and greater driving confidence.

Ever tried to stop quickly only to have your foot hit both the brake and accelerator pedals at the same time? Or tried to work the pedals with winter boots that overlap them?

That might have caused the vehicle to bang to a stop or lurch forward--or both.  Either way, such an experience can be disconcerting and disorienting.

Safety First--STOP!

The Subaru Brake Override System (BOS) is intended to help prevent such situations, which frequently result in accidentally accelerating the vehicle.  For BOS to function, both pedals have to be depressed when the vehicle is moving faster than 6 miles per hour.

The vehicle's computers monitor the engine's electronic accelerator and the operation of the brake.  BOS compares data from both systems. Using that, it determines if both have been applied simultaneously and if the accelerator is pressed down 80 to 100 percent of the way.  If so, BOS still doesn't immediately spring into action. Instead, a built-in delay gives the driver time to react by lifting his or her food off the two pedals.  If that doesn't happen, BOS takes over to stop the vehicle.

BOS Overview

After BOS activates, it raises the pressure of the break fluid high enough to stop the vehicle. This is managed by the Anti-Lock Break System (ABS) electronic control unit. BOS also closes the throttle plate to reduce engine speed. At this point, the vehicle cannot accelerate. The accelerator will resume functionality once the driver has fully released it.

One of Two Stages

How quickly BOS operates is a function of how heavily the driver pushes on the brake pedal.  In situations where the brake pedal is lightly pressed along with activation of the accelerator pedal, the system functions with a delay.  As a result, the driver still retains control over the accelerator with a brief tap of the brakes.  However, if both pedals are hit heavily at the same time, BOS activates immediately.  Acceleration yields to braking and the vehicle stops.

Back to Normal

BOS deactivates when the driver operates the brake and accelerator pedals separately.  The ABS control unit releases brake-fluid pressure. The throttle plate is electronically positioned so the engine runs above idle to prevent it from stalling.  BOS also intervenes with engine operation to prevent the vehicle from lunging forward.  Finally, BOS synchronizes the position of the accelerator and the operation of the throttle plate so the vehicle drives normally.

Enhanced Safety

By helping to prevent the driver from accidentally accelerating the vehicle, BOS contributes to safer driving and greater driving confidence. This Subaru safety system is standard on 2013 BRZ, Impreza, Legacy, XV Crosstrek, Outback and Tribeca models, and it will be standard on all future Subaru models, including the soon-to-be-released 2014 Forester.

To learn more about Subaru Brake Systems, including the Anti-Lock Brake System, Brake Assist, Electronic Brake Force Distribution System, see "Subaru Brake Systems".
Source: Subaru Drive Magazine Winter 2013 Issue. http://drive.subaru.com/win13_WhatMakes.aspx