Do you have a specific example of this?
Waze will learn the speed of that road, and will take it into account when considering possible routes. It generally prefers to use major roads where possible.
If you feel that most vehicles should completely avoid using it, then 'Dirt Track' is an appropriate type to use. There's an option in the client to avoid these kinds of roads for less adventurous drivers. However, if the road is proper tarmac with passing places, it should probably remain 'Street'.
HQ is considering adding Vehicle Types
to their app, so that large vehicles could be instructed to avoid single track roads.