I think you're in the very advanced problem zone =P
How are your roads placed? Are there constraints like do they need to be connected at ends only, is there a limit to how many roads per intersection?
When you create an intersection, you'll need some information there including which direction each option is in, and how long that road segment is.
Basically every time you place a road, you will need to add information at the intersection it was placed on.
You'll also need to create your own pathfinding - A "simple" pathfinding algorithm would find every possible path to the destination, and compare the total values (length?) of each segment of road to pick the path with the lowest total number. This algorithm should record which road to take at each intersection in a variable or array, which the car then references to make its drive.
Given that your roads are straight, you can have the car angled at the same angle as the chosen road and have it move forward until it hits another intersection.