I think some visual exhaust effects as cars change gear, or engines overrun, would add to the sense of movement/speed and even though these effects may not be strictly true to life I think they would help with the immersion factor (I think the dust in the rallies does this for instance). Also you would possibly also get a better indication of when others are changing gear or lifting off the throttle and be more prepared to react.

Forgive me if this is a naive idea but I am new to this and play (practice) mainly in single player mode where on tarmac staring down the dead exhaust pipe of a VI driver lacks excitement. However the same can not be said for racing off road on the rally courses where the dust creates a whole new feel to the experience.

Perhaps if this idea is not practical - or within the true nature of LFS - then dust and dirt on the circuits (as I believe has been suggested elsewhere) might have a similar effect.