OK, played it through, my crits (don't take this too harshly, if this is your first map it by far outshines my first map, andmost other people's too):
1) If this map is finished it should have a level preview and materialsequence screenshot (or at least a static one)
2) You should choose some backing music for it. I believe that the racing always feels better when you can hear music along with it.
3) The shadows are far too dark (read: black), I suggest adding a zoneinfo and setting the zonelight ambient light properties to a grey-blue colour.
4) The light is too blue, there are two points associated with this: The light actually makes it harder to see after a while because it is too saturated and also it looks silly with the red background anyway.
5) The tunnel bits have bumps in the floor that the motorcycle and side car can't get over at all.
6) The buildings are very cubish.
7) The texture for the walkway-style thing is very blurred. I suggest using a texscaler and scaling it down at least 2x.
8) The meshes on the top of the buildings look very randomly placed, and randomly chosen. As far as I know they serve no purpose (Think: If I was building a skyscraper what would logically go on the roof: a bunch of random pillars or some air-conditioning vents, etc.)
9) I got lost a couple of times, here Steve's arrows would really help
That's all I can think of. Anyway as I said don't take it too much to heart, as a first map it's pretty good, and I'm only saying this so that your second map will be that much better.
BTW. That firework effect at the end is good, but maybe some soun to go with it?.