Currently within the app a decision has been made as to which roads to include and which not to. A farm dirt track is probably not included, as is a small B road. To me it seems that to distinguish between major and minor roads would be not significantly different. An app setting that gave the option to only display dual carriageways (or motorways), or display only A-roads, instead of all roads (above the aforementioned theshold), may make the app more user friendly in some situations.

For example if I fly in Gloucestershire I have the roads showing because the major roads are useful line features, whereas if I fly in Essex I turn the road display off because there are so many roads it becomes fairly meaningless. I find generally, unless flying a helicopter looking for a local address, the smaller roads are not very useful and difficult to use for orientation. In an aeroplane at or above 1500 ft, it is only the major roads that are of use when navigating.

This is of course only my opinion.