This feels like a really solid concept! Splatoon meets Need For Speed.
It's already been said, but a smaller map would probably make it better. In a multiplayer game you don't want players going off and doing their thing alone. Multiplayer is best when interacting with other players. And in a smaller map the players will naturally spend more time bumping into each other and going over each others' tracks. The size of the current map feels like you could each have your own single player adventure separately (which isn't great).
I haven't played it properly in 2 player yet. But I feel like there's an obvious mechanic when it comes to bumping cars:
The opponent's car should not explode, instead it should spin away, out of control, leaving behind a trail in your color.
Another thing, that has been mentioned. There should be a way to get nitro back... perhaps skidding slowly replenishes nitro? (I guess it's that, or over-time, or pickups). Hmmm... Are those blue capsulte pickups?
I don't play many driving games, but I enjoyed the extra-arcady feel while I had Nitro. Felt like I had more control than in a normal driving game (and the Nitro made it much easier to keep a skid going, which felt fun).
Getting stuck upside down after ramping was less fun :P