http://forum.paradoxplaza.com/forum/showthread.php?845936-Request-Precise-road-height-adjuster
Precise road height adjuster:
Default Road height tool:
Press UP or DOWN to raise or lower roads at big increments
Mod Request:
Hold MODIFIER key - your road is locked in place (on X-Y-coordinate) and now moving your mouse adjusts road's Z-coordinate (height)
Release MODIFIER key - return to normally adjusting X-Y coordinates, but with the Z-coordinate you've just set