Re: dirt vs street
Try contrasting the pace and preditability of a trees movements verses a car, truck, pedistrian, pets and/or wildlife.
Change of road conditions; planks of 4x2 on freeways, (happend two days ago to a member of a local club), oil slicks, gouges in road surface on corners, etc
If you fall off its not soft dirt you hit, you may even be lucky enough to find some of those nice wire barriers, lovingly refered to as 'cheese graters' by the local riders.
speed cameras etc
If he refuses to listen, then just go with the "fair enough, but its your funeral" line and see if he takes stock then.