Yeh thanks Viper ,I have a magazine called PSW with a preview of GT4 an another interview with Yamamuchi-san! I cant scan things at the mo otherwise I' put it up here too.
In the article it does talk about a system for when you run on the grass where by a black flag appears on screen and you lose power for a short while. Sounds a bit cack to me .
On cutting corners K.Y. says: "When you stray too far from the track, a black flag will be displayed. Players will then notice that for a certain amount of time there will be a speed limiter applied to their vehicle..."