I don't really like the border, the white seems a bit blurry? And it doesn't make sense to my eyes to have each car as a focal and have them so far apart, and then blur the rest of the background. I think there should be just 1 focal. The fire effects are really nice, the dust effects are good but i think in the wrong places. I also think you shouldnt have an even blur on the entire road, some parts should blur more, probably the areas closer to the foreground.
Tried a new border, shouldn't be blurry, maybe the contrast is making it look weird. The two focals are always a challenge to do, which I why I wanted to try it, not only that but the cars were too close together to do just one as a focal. Don't know about the blur, seeing how it's all one speed it should be the same/even...
Just to quickly point out, ground closer to the image would appear to move faster. If you are focusing on the car, it is as if everything except the car and you is moving, or if just you and the car are moving. Either way, the ground closer to the forground should appear to move more violently to give a better effect. That's just by physics mind saying that though, it's not what will necessarily look best. Fair points about the rest, the burry border is a bit of an optical illusion with the contrast between black and white, and then white and the image. If you fancy it, i would probably have a go at focusing the motion blurs into the top right corner instead of the bottom right? Just something that might prove good. Other than that, it's fab, really high quality image and effects.