Great gameplay! Make it longer! Here's how:
Here's how you take a quick mini-game like this one and turn it into a full-fledged Casual Game:
-Separate the gameplay into discreet stages. (Each stage should have vastly different backgrounds, and one slightly unique twist on gameplay, such as motorcycles instead of cars, or stationary buildings you have to drive around.)
-Make some kinda map screen so the player can replay old stages and unlock new stages.
-Keep a local "top score" for each stage, that represents the player's personal best.
-Add a level-up system that the player can use to "spend" points to improve the capabilities of his vehicle.
-Save these per-stage top scores (and other persistent player stats) to the user's hard drive as a saved game file.
Once you've implemented all of those features, you'll need to balance the game:
-Give the player a way to be a daredevil. That is, let him take on more challenges if he thinks he's powerful enough to get away with it. Let the player up the ante at any time, making the game harder but also increasing the potential rewards.
-In order to beat the last stage, the player will almost NEED to buy all the persistent powerups.
-Achieving the highest possible score in all the stages should give the player PLENTY of points to buy most of the powerups.
-You should be able to beat most of the earlier stages even if you have only minimal powerups.
-A high-level character should be able to replay the earlier stages and, because he's so strong now, be able to take on more challenges and therefore achieve a MUCH higher score than before.
And yes, I stole all of these ideas from GemCraft. :) It's a tower defense game, not a driving game, but it uses a lot of game design ideas that are just good sense for any casual game with quick, simple gameplay.
If you implement all of the ideas above, you'll end up with a long game with plenty of challenges all the way through, wherein the player has total control over his actions, but he gains a slight advantage after every stage, and over time he can be more and more of a daredevil, constantly improving on his previous best scores and learning new obscure tricks towards getting the best scores. And, yes, in this case the scores would actually mean something tangible then just competing with the 20 most hardcore players on the internets, it would be how you level-up over time. :)
You can get away with just a fraction of these things, of course. Just make the game end not when the player finally can't keep up with the escalation and runs out of time, but when the player finally beats the last stage. That alone makes the game more rewarding. "Can I beat the game" is just psychologically much more fun than "How long can I last?" That and some kinda persistent level-up system instead of an oldschool collectable powerup system. Those two things will lend any decent mini-game considerable replay value. You don't necessarily need to copy GemCraft, it's just that GemCraft did a lot of things really well.