But once the vehicle is moving, the engine will not shut off unless the button is held down for a full three seconds -- a period of time in which Saylor's car would have traveled 528 feet. A driver may push the button repeatedly, not knowing it requires a three-second hold.
"When you are dealing with an emergency, you can't wait three seconds for the car to respond at 120 miles an hour," said Clarence Ditlow, executive director of the nonprofit Center for Auto Safety.
The ES 350 Saylor was driving that day was a loaner provided to him by Bob Baker Lexus when he took his family's Lexus in for servicing. It's unclear whether Saylor's own car had the same feature or whether he was aware of the shutdown procedure. Bob Baker Lexus did not return calls.
That procedure is explained deep in the owners manual. In a text box labeled "! Caution," Toyota tells owners, "Do not touch the 'power' switch while driving." But under the warning it adds, "If you have to make an emergency stop, press and hold the 'power' switch for more than three seconds."
Lyons, the Toyota spokesman, said: "I think the text is valid. What I'd prefer it to say is to explain that you'll lose power assist [for] brakes and steering if you do so."
The shutdown procedure reflects a larger problem: As auto manufacturers adopt increasingly complex electronic features, it becomes more difficult to explain how they work, said Paul Green, a human factors expert at the University of Michigan's Transportation Research Institute. A study by the institute found that in some cases, owners manuals would have to run up to 1,000 pages to fully disclose everything.
"In the past, systems were pretty simple," Green said. "You put a key in the lock and turn it. Now we have a fob with functionality."
The other common defense tactic advised by experts is to simply shift a runaway vehicle into neutral. But the ES 350 is equipped with an automatic transmission that can mimic manual shifting, and its shift lever on the console has a series of gates and detents that allow a driver to select any of at least four forward gears.
The arrangement of those gear selections could make it difficult to shift from a forward gear directly into neutral in a panic situation, Toyota spokesman Lyons acknowledged.
"I think it's possible to get the shifter confused, but I can't be sure that's what happened" in San Diego, Lyons said. "You'd be surprised how many people around here [Toyota] don't know what the neutral position is for."
The most obvious impulse for any driver experiencing sudden acceleration is to apply the brakes. But when an engine goes to full throttle and is speeding at 120 mph, the brake might not stop the car.
The ES 350 and most other modern vehicles are equipped with power-assisted brakes, which operate by drawing vacuum power from the engine. But when an engine opens to full throttle, the vacuum drops, and after one or two pumps of the brake pedal the power assist feature disappears.
As a result, a driver would have to apply enormous pressure to the brake pedal to stop the car, and if the throttle was wide open might not be able to stop it at all, safety experts say.
"I don't think you can stop a car going 120 mph and an engine at full throttle without power assist," said Ditlow, the safety center director.
Indeed, a 2007 study by federal highway safety officials showed that braking distance and force on a Lexus ES 350 increased fivefold when the throttle was wide open. And evidence introduced in sudden acceleration trials suggests that it can take up to 225 pounds of pressure on a brake pedal to arrest a runaway vehicle, far more than most drivers can muster from a seated position, said Edgar "Hike" Heiskell, a Charleston, W.Va., attorney who is suing Toyota over a fatal acceleration accident in Flint, Mich.
Lyons acknowledged that the vacuum can be depleted when an engine throttle is wide open, leaving the drivers without power-assisted brakes.
"There's a [federal] standard where you have to be able to stop the car without power-assisted brakes, but obviously I don't think it includes situations where the throttle is wide open," he added.
Drivers in other crashes also found it difficult to rein in a runaway Toyota. Guadalupe Gomez of Redwood City said he was held hostage for 20 miles on a Bay Area freeway by a 2007Camry traveling more than 100 mph.
Gomez was unable to turn off the engine or shift into neutral and then burned out his brakes before slamming into another car and killing that driver, said attorney Louis Franecke, who represented that victim's family.
The San Diego crash is still under investigation by the San Diego County Sheriff's Department and the CHP; until the probe is complete, neither agency is commenting.
The National Highway Traffic Safety Administration, meanwhile, says it has an open investigation into sudden acceleration events involving Toyota vehicles.
Times staff writer Tony Perry in San Diego contributed to this report.