In my opinion this effect must be added! Of course people will be able to enable it or disable it, but this must be in the game, so you can feel the speed when driving at high speeds and falling from a cliff. I think it's a good idea
I think it could be done with PostFX shaders, I googled around and found this http://www.garagegames.com/community/forums/viewthread/67131
There is motion blur, but it doesn't work with the car movement, only with camera movement! Type this in your console (to open console press ~): Code: MotionBlurFX.enable();