Thanks for the feedback Jamie. We'd love to be able to do this but are a bit restricted by the mapping data available. If it becomes possible we'll certainly look at it. There is a feature request for this that you can vote for if you like (number 8 on our Feature Requests list).
would be nice to be able to exclude certain types of roads. I've found myself a couple times after plotting a route coming to a dirt road. Not the best for a sportbike...