Unfortunately, this is a by-product of the limitations of using RGB color to calculate a heightmap.
The bas relief converts the specified texture to grayscale before creating the heightmap. This means there are only 256 possible values for the Y axis.
Most images do not have a very smooth transition from one pixel to another, so you end up getting a "stepping" between pixels.
One fix for this would be to convert your original image to greyscale (if it isn't already), then use an image editor to smooth/blur the image so the differences between pixels isn't so great.
The problem with this is you lose details between lines (in your case the roads would merge with the surrounding landscape). If you have an image editor that can selectively blur areas, that may be a workaround.
Not the most ideal solution, but aside from adding a smart smoothing algorithm to the bas relief, I'm not sure what any other options would be...
Edit: If you end up going the image editing route, you can get a free image editor at
www.gimp.org