I have used Game Maker before and have a license key for it, but I hardly ever used it and don't at all anymore.
Before you get any further (if you do), consider using a programming language such as Python instead of using GM. C++ would be faster than Python, but Python is much easier to learn than C++ and there will still be a noticable speed increase in the game. You then have the option of running the game on Linux and Mac. Game Maker is an alright tool if you want to get the feel of the game developing process, but you shouldn't use it permanently. It's simply too limited to do things you'd want to do later on. Plus, most of the people on the GMC are stupid.:thefinger Sure, there are some pretty talented people there, but MOST of the people are still idiots.
The game has crappy midi-sounding music, but it does not mean the game sucks. Let the cars have speed increases over time. It seems like too much of a slow paced game to be like GTA. If you drive your car in reverse, it doesn't kill the person. It seems they only die if you drive forward into them.
Once you make the cars to where their speed increases, also think about making it so you can slide with sharp turns, and then you'll need it so being hit by the side of the car will also kill the person.
It has kind of cheesy sound effects, but I don't think 2d GTA sound effects were very good either.
As much as Mark Overmars has added new features to game maker, the biggest limitation is that it's not portable. And I don't think he intends it to be. So anyone using a decent OS is out of luck and will not be able to play your game. Choose a better tool.