A few suggestions:
1. Only 1 pixel of tarmac between the white border and the grass
2. Longer S/F straight
3. Bigger gaps between sunk tyres
4. Longer kerbs - I mean in the last corner 7 yellow/red squares would've been better than only 5.
5. Use more space of the "sandbox" for layout
6. Zoom (almost) as close as possible
7. Wider pit lane
8. This is just a matter of taste, but personally I prefer the "border" color and mud more as dominant colors in dithering, instead of sand
9. Has been mentioned earlier but could be bit wider. Personally I like the track to be a little bit more narrow than bridges, but this is slightly too much
It's a nice track, but could be better
EDIT: I noticed that the borders aren't perfect, so I'll tell how to make them very fastly and easily in Photoshop (AFAIK you have it) in case you don't know this trick.
1. Select track with magic wand (you must have anti-alias off)
2. Go selection -> modify -> contract 1 px
3. Go edit -> stroke (outline) selection -> don't change any options, choose white (or yellow) as color, click OK.
4. Enjoy your borders made in 15 seconds