Since there currently is no way to render the shadow on an invisible surface in Microstation - I've made this tutorial for a simple work-around:
Thanks for the comment Kurt, I was trying to do exactly that (render a shadow on to a surface which is not 'Visible to Eye') a day or so ago. The pick boxes do tend to suggest it should be possible don't they? No feedback from Tech Support so I assume this is still not possible without post production.
thank's for the suggestion. I don't have any posting priveleges in the Wiki section. Anyway, I guess there will be a Luxology update soon that will make my suggested work-around obsolete.
Make it a Wiki in the Vis Group.
This is too good to keep it just here.
btw: thanks for your comments guys!
you can define the asphalt plane as invisible to eye, and render out the alpha map - no manual cutting neccesary. There are some antialiasing issues when using this method though...