You have to create the road simulation ...
a little array with average positions, for example a road with turns you have to get the A max and B max positions so you won't drive off the road... also you have to drive without crashing into cars so you have to make smart decisions...
this is gonna be a long process, so just don't do it... it is not worth the time
OR you can just be ignorant and set the direction towards the position you want the car to go, and then calculate the angle and set it, then the car will just go to it, surely it will crash on cars and sometimes will get stuck on buildings...