Sorry Frank, you've previously given the impression that you didn't like getting into math, hence my comment. My bad. I've got to say I really like the modulo function, it gives you all sorts of possibilities.

Those circular artifacts tend to occur when you displace/warp something outside it's scale range. eg a perlin set at say 100m scale and you multiply that output by something a lot larger than it's scale, like 200m.

As for 'smoothing'. What I meant by that, was some way of looking at a defined area (say x +/- 10m by z +/- 10m) and finding out an average value of y, then smooth step between these values of y to give you a flat road surface. You could use this for rivers down slopes, etc. The only way I've seen this done, and it's not a perfect solution by any means, is by smoothing a heightfield masked by the limits of your road/river.

Cheers

Jon