Make actor move on roads

I spawn a roadmap, by reading a xml file with location of different road section. Then straight section curves and such are spawned using Hierachical Instanced Static Meshes. Now i want small cars to move across the roads systems. The roads are build at runtime and the maps vary, so i have not been succesfull using simple splines.

Basically the cars should drive straight if it is at a straight piece location, then turn left/right when encountering such sections.

Any ideas on how to tackle this issue ?

How do i make small cars drive across these roads without controlling them myself.