Create a Blueprint

In a graphics editor, open the height map you plan to use for reference.  Add a layer above it.  Paint your roads on the top layer in white with a 12px brush to match in-world road size.  Add outlines of lots.  A 64x64 lot is 64px square, and so on.  Keep in mind that you will need quite a bit of room to smooth hils near roads and that lots require some space between.

You'll need to flatten the image for export, so save a copy of the version with layers for editing.  For the exported image, you'll need a 100% black background behind your white blueprint, and the height map (if it isn't pure black) should be hidden.

Import the blueprint as a terrain layer and give it a texture that will stand out against your other colors.  Ta-da!  You'll see exactly where you want to place roads and lots.  When you are done, simply delete the terrain layer.  Making additional blueprints to map out things like lakes, unroutable high-density forests, or signs can also be helpful.