What's the difference between the A.B.M. highway in the UK ?

m is a highway, with one-way three or four lanes. a is similar to a national road, but in fact there is not much difference from b. Only a few sections are one-way dual carriageways, and most sections are single carriageways. The quality of b-grade roads is roughly the same, except that a-grade roads are slightly wider under certain conditions. Anyway, driving on both a and b is tense, mainly because the locals drive too fast.

m is better than a, which is better than b. The fewer numbers behind the letters, the higher the road grade. M level is the best, equivalent to a national expressway, but the road conditions are much better than domestic expressways; a is equivalent to a national highway (usually one single lane in each direction, for example, the A9 we took back from NC500 to Edinburgh is a single lane in each direction), and the road conditions are also better than domestic national highways; sometimes a and m are combined, such as A47 (M), which is a good dual carriageway; b level is basically one single lane in each direction (there is only one lane, and overtaking has a passing place), but the road conditions are much better than domestic roads (the road we took in the highlands of NC500 this time is very good).