OK guys... First off, I am an ASE Certified Master Technician with over 15 years in automotive repair.
Yes MOST cars built today do in fact have NO mechanical connection between the throttle pedal and the engine. In the old days, a cable connected the gas pedal to a valve that controls how much air enters the engine. The computer of a typical fuel injected system would detect this by a throttle position sensor at the air valve at the engine, and an airflow sensor (or a pressure sensor, or both) and would command the fuel injectors to stay open longer to add more fuel, allowing the engine to 'rev up'. There was a separate air valve (often called a idle control valve) that was electrically controlled that would allow air to flow around the mechanical valve so the computer could control engine idle speed... to compensate for temperature, engine loads (A/C compressor, Power steering, high electrical loads, etc), and driving conditions.
On Most cars built today, instead of a mechanical air valve, it is controlled electrically by the computer with an electrical motor. This 'simplifies' things a bit as the idle control valve can be eliminated. The air valve is under a heavy spring load that the electric motor has to overcome so in the event of just about any failure, the computer would simply kill voltage going to this motor, and the air valve would simply spring shut. Basically, the engine would likely continue to idle but depressing the gas pedal would result in nothing happening.
There is still a throttle position sensor at the air valve at the engine, but there are also two throttle position sensors built into the gas pedal assembly. The computer uses the two in the gas pedal for redundancy purposes, and it uses the sensor at the air valve to make sure the electric motor is moving the valve to the proper position. If the two pedal sensors don't read the same, the computer sets a fault code (check engine light) and will uses the lower input of the two. If the computer sees a failure of the engine sensor (or the circuit for the throttle motor), it sets a fault code AND goes into a 'limp mode' of sorts which basically disables the throttle.
This makes it almost IMPOSSIBLE for the electrical system to malfunction in a way that the throttle valve at the engine would stay in the open position.
Now, here's the problem with the Toyota systems. The Gas pedal assembly has an amount of built in mechanical resistance built in to it, to make it feel almost identical to the older cars which had resistance in the pedal due to the cable and mechanical valve. What is happening is the pivot point of the pedal is wearing down a bit, and the resistance 'plate' rubs the pedal too hard, making it physically stick in a downward position. Imagine your joystick... if you spilled a soda on it and it got all gummed up so bad that it would hang up on the gunk so much the spring couldn't pull it back to center. Same basic thing.
Since the Pedal is sticking mechanically, the sensors all read properly and the computer actually thinks you're pressing down on the gas pedal.
The fix is to install a shim into the pedal assembly to push the resistance plate further away from the moving parts of the pedal, to compensate for the wear in the hinges.
Skuzzy is incorrect about traction control never having a mechanical connection. Even if you hold the air valve open mechanically, the computer can still kill the injectors, or the coils to prevent the engine from running, or to reduce its power.
Skuzzy is however right about every manufacturer having skeletons in their closets. I probably don't need to mention the Pintos.. or the 2.6L Mitsubishi engines in the first generation Caravans... Or the Ford Probe... Or the plastic intakes on the Ford 4.6L... or the plastic intake gaskets on the GM 3.1L, 3.4L, 4.3L, 5.7L... or maybe even Dexcool (known in the industry as 'death-cool') antifreeze. Neon Head Gaskets? How about the Ford trucks massive cruise control switch recall... and then the recall on their recall... that recall which was revised over 17 (yes that's SEVENTEEN times) because the harness that installed a fuse into an unfused circuit, had the fuse on the GROUND SIDE of the circuit for some models... Anyway.. I could go on and on and on. And yes imports are NOT exempt from these sorts of things either...
Regardless, with this being said, should your car even continue to accelerate after you take your foot off the gas, there ARE things you can do. If it is a stick shift, you can always depress the clutch. In an automatic, you can always move the shifter to neutral. In either case, you can simply turn off the key (or if no key, push and hold the START/STOP button).
I just had a heated debate with my g/f a couple hours ago about this. She said that she had no idea one could turn off the engine while driving the car. I asked her if she ever read the owner's manual for her car. She said 'no'. It really amazes me how people will spend hours studying the manual for a $25 piece of electronic junk (or a $300 Joystick.. you get the point), but when it comes to a $20,000+ vehicle, most people never even look at the cover. Yes it's very easy to blame the manufacturers for accidents resulting in malfunctions such as this, however if people would take even a few minutes to learn about these machines that they are trusting their own lives (and the lives of their families) to, then the VAST MAJORITY of these accidents could be easily avoided.
