First, sorry for lack of feedback on last version of oblige, but you gave me very little to complain about so... Will have feedback soon.
But this sounds like a cool project. First before I forget, TORCS is an open source racing "game" that was made, if I am correct, for "racing AI" competitions. The track layout files are very simple and can be edited in a text editor, though the actual track model is separate, and I know less about that, as there is a program for converting the track layout into a model. The advantage of this program is that the AI's can race custom tracks without any fiddling about, and are very tough to beat. It is however punishingly unforgiving to race even with an analogue joypad, but maybe take a look.
If you are making track for nfs1, then the layout does not have to be a circuit. The reason I bring this up is because I have an idea for how I might go about generating a track, but it will not produce a circuit.
First you need information about the acceleration of some benchmark vehicle. You also need a function that converts the current speed of the vehicle into a turning circle. Then you simulate the vehicle accelerating across an empty field, and draw the track to follow the vehicles path. When the program decides to put a bend in, it looks at vehicle turning circle, and if it's an easy corner, it will be less than the tuning circle, hard will be more, though the funnest corners will be very near to the vehicles turning circle (I reckon). If the corner is sharper than the turning circle of the car, the simulated cars speed is reduced to a value where its turning circle matches the steepness of the turn.
To really make this work there needs to be some calculation involving the width of the track, as even a sharp turn on a very wide track will be easy to tackle at high speed, unless the corner is very long. Also you would need something to stop the track overlapping. Also hills and road camber will have massive effects on the speed at which you can take a piece of track, and you must avoid putting corners after jumps!
The deformation idea sounds pretty cool, and you could try lots of different types of deformation, find an article on deformation, or types of symmetry, and go wild.
Also really abstract, recursive, fractal type things might be cool to play with. The dragon curve with a low (30-40 degrees) folding angle looks like a particularly tricky track, or maybe you fancy a lap around Gosper island.
You could also produce tracks by adding lots waves together with different amplitudes and wavelengths (and phase), converted into angles for the track to change by, or forming the waves into a ring, and make sure all the wavelengths are factors of the ring length. (This is basically additive synthesis)