Though NASCAR officials say they cannot be certain of the cause of two accidents which claimed the lives of drivers Adam Petty and Kenny Irwin in the past three months, they took steps Tuesday they hope will help prevent more tragedies.

NASCAR announced two safety changes to Winston Cup series cars that will take effect with Saturday's Brickyard 400 at Indianapolis Motor Speedway.

Technical bulletins were issued this week to all Winston Cup teams mandating each of the primary and secondary throttle shafts of every car must have an "independent travel stop" that would prevent the throttle from remaining wide open.

In addition, NASCAR is also mandating each car be equipped with additional ignition on/off button that will disconnect power to the ignition system. The button must be mounted on the steering wheel within reach of the driver's thumb and it must shut off the engine immediately when depressed.

The changes come in the wake of the deaths of Petty on May 12 and Irwin on July 7 - both of whom were killed in accidents at New Hampshire International Speedway. Both drivers wrecked in Turn 3 of the speedway in accidents thought by some to have been caused by hung throttles.

Petty was at the track for a NASCAR Grand National series event, while Irwin's accident came on the weekend of the track's New England 300 Winston Cup race.

Last month, NASCAR announced that its investigation of Petty's accident didn't turn up any evidence of a hung throttle from an inspection of Petty's car, but it still couldn't rule out the possibility.

Kevin Triplett, NASCAR's director of operations, said a similar inconclusive finding was made following an investigation of Irwin's accident, but NASCAR thought the safety measures were a good idea nonetheless.

"In the work we did, and if you want to use the word investigation, we didn't find anything conclusive (in Petty's case) and trying to reconstruct things is one of the most difficult parts, one of the most difficult things we face," Triplett said.

"But even though there was nothing conclusive, through conversations in the garage area through our talks with crew chiefs, drivers and car owners and the time that we spent in those conversations, this was still an area that we felt like something could possibly be done. We hope that this is a step in that direction."

Triplett said the decision to go with these particular safety measures were made in part because several teams had already experimented with them.

"The throttle stop was something that several teams were using already. The auxiliary ignition switch or on/off switch is required on the dash of every car anyway," he said. "This is an auxiliary switch that is on the steering wheel.

"In trying to find something that was effective across the board and comfortable across the board, these were the two areas that were easily done and could be done before this weekend."

Reaction in the series from drivers and crew chiefs was positive, though some teams were caught off-guard by the announcement and had to work this week to get their cars fitted with the new equipment in time for Saturday's race.

"It's a good thing," said Jimmy Fennig, crew chief for Mark Martin's No. 6 Fords. "At least NASCAR is attempting to try something for safety. It will be good for the better of the sport."

Winston Cup driver Jeremy Mayfield said he didn't see the changes causing any problems for drivers or their teams, just solving some.

"A lot of times, you can make rules changes and, next thing you know, you have a whole bunch of other problems. The restrictor-plates are a good example. They slowed the cars down but they opened up some new stuff to deal with - closer racing with larger packs of cars and stuff like that," Mayfield said. "The slower speeds probably made the racing a little safer, but they made it a little tougher, too.

"When you're running these things at around 200 miles per hour, there's no way you're going to make them totally safe. There just isn't any way to do that. But anything that makes the cars safer, especially something that doesn't affect the racing and something that doesn't open up a can of worms for other stuff, it's just smart thinking."

Triplett agreed.

"In looking at these things, you have to try to find things that will create a help without creating another avenue to try to get around," he said. "In the two years of research we spent on the roof flaps that evolved from deck lids and everything else, one of the main concerns we had was trying to create something that would be a safety device that wouldn't impede anything that the teams were trying to get around.

"We had to show the teams that the roof flaps would only activate in a spin or something like that, they wouldn't deter the teams from going faster when a team was racing. You don't want something that a team would be tempted to bypass because that's the whole opposite of what a safety feature is for."