In 2009 Leeds university tested for 6 months cars fitted with speed limiters, the first 3 months the limiters was active. then the last 3 months was turned off. so they could compere the differences. I believe that the findings was it was not suitable to implement them in cars for several reasons. I was one of the drivers of such vehicle a bloody Skoda fabia http://www.leeds.ac.uk/news/article/91/speed_limiters_tested