[mkgmap-dev] Reduce road class/speed when road is narrow

I want to dissuade my gps from using very narrow roads (for car routing) so one way of doing that is to reduce the road class and/or speed if the road is narrower than some threshold. I can easily imagine how to do that in mkgmap using some new Java code but I suspect that it could be done using style rules (of which I know nothing). Is that possible? Thanks, Mark