Having just finished a vehicle turning path object, the question that needs asking, is what functionality is truly needed? As in, do you need cars only or does it need to go as far as articulated trucks/buss'?
Having had to learn the physics behind it all, I can tell you the jump from rigid to articulated vehicles is a HUGE one!!
To be honest, for most people the vehicle turning templates are good enough.
And an add-on is massive overkill because it can be handled in a 2D object that gives simple, understandable visual feedback! (Plus reissuing an add-on every Archicad version has it's own issues, historically)
That's just my two cents anyways.
Ask me about rocket trajectories some time!