US Transportation Secretary Ray LaHood said: 'We enlisted the best and brightest engineers to study Toyota's electronics systems, and the verdict is in. There is no electronic-based cause for unintended high-speed acceleration in Toyotas.' 

The Department of Transportation added that the two mechanical safety defects identified by NHTSA more than a year ago – “sticking” accelerator pedals and a design flaw that enabled accelerator pedals to become trapped by floor mats – remain the only known causes for these kinds of unintended acceleration incidents. Toyota has recalled nearly eight million vehicles in the United States for these two defects.

Responding to the NHTSA study, Steve St Angelo, Toyota's chief quality officer for North America, said: 'Toyota welcomes the findings of NASA and NHTSA regarding our Electronic Throttle Control System with intelligence (ETCS-i) and we appreciate the thoroughness of their review. 

'We believe this rigorous scientific analysis by some of America's foremost engineers should further reinforce confidence in the safety of Toyota and Lexus vehicles.  We hope this important study will help put to rest unsupported speculation about Toyota's ETCS-i, which is well-designed and well-tested to ensure that a real world, un-commanded acceleration of the vehicle cannot occur.'

The NHTSA added that, as a result of its findings, it's considering taking several new actions for all carmakers, including:

• To propose rules, by the end of 2011, to require brake override systems, to standardise operation of keyless ignition systems, and to require the installation of event data recorders in all passenger vehicles.

• To begin broad research on the reliability and security of electronic control systems;

• To research the placement and design of accelerator and brake pedals, as well as driver usage of pedals, to determine whether design and placement can be improved to reduce pedal misapplication.

'While today marks the end of our study with NASA, our work to protect millions of American drivers continues,' said NHTSA administrator David Strickland. 'The record number of voluntary recalls initiated by automakers last year is also very good news, and shows that we can work cooperatively with industry to protect consumers.'