The race tracks, not just the straights but the corners. Just to give you an example, it's like racing These cars on little gokart tracks or in parking lots:
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They are just too fast for any real form of racing to take place on a small narrow track.
The race tracks need to be enlarged further to reduce the relative sense of direction changes, gives the drivers more time to think and react.
I don't know if you play videogames, but i think with icreased power and handling F1 should aproach F-zero and wipe-out like race tracks for increased excitement.
To stay on topic though, removing the DDD is just one small step, it won't eliminate the wake completely. I think this is what Horner was alluding to. The disturbance behind the DDD only falls in the silhouette of the already dirty air coming from the wings, rear wheels, body and diffuser.