I’m quite comfort with the present guidelines in wiki, although some point should be slightly edited. However, I will explain my idea of principal of highway classification.
The main point here is “What is the main purpose of highway classification for map usage?” In my view it is for “route selection” for best navigation. The higher class of highway should be faster for travel between two point in compared to similar way with lower classification.
First, I suggest to separate National Highways and urban streets, since in Thailand National Highway is mostly out of urban area (former national highway that pass through the urban area have mostly transferred to local administrative organization, and the highway number is then be removed). So National Highways and urban streets should be under different guidelines.
For National Highway, I think we should avoid highway number at all. Since nowadays highway number is very confusing and not reflect the principal of highway classification.
If we look at the principal declared by the Department of Highways (DOH), as link: http://www.highwayweigh.go.th/highway_information/types_of_highways.html
To sum up, 1-digit is highway from Bangkok to each region, 2-digit is main highway in each region, 3-digit is main highway between provinces, and 4-digit is main highway within provinces. Classification like this looks good, but unfortunately in reality it’s not strictly follow these definition, not to mention Rural Road maintained by the Department of Rural Roads (DRR) which some of them is more important than some National Highway.
If we focused at that definition, and don’t mind whatever number of digit defined from DOH, the highway classification for map usage is making more sense.
Let’s make it match with OpenStreetMap tag, besides motorway and trunk which focused more on characteristics than importance, the other class should be classified as follow:
- Highway from Bangkok to each region: since it meet trunk requirement so this is not necessary to make any specific guidelines.
- Main highway in each region: most part of it is trunk too, so the others that not meet trunk’s requirement should be primary.
- Main highway between provinces: I think we should slightly edited, since some provinces is very large and contain many cities, if we focused on “city” than “province”, it maybe make more sense. So, main highway used to travel between cities should be primary.
- Main highway within provinces: secondary.
- Rural roads: tertiary.
(secondary and tertiary shouldn’t be identified by who is the owner, DOH or DRR, but should be identified by importance. Maybe keep in mind that higher class is better and faster way between two points.) - Road within community: unclassified, residential, or service.
Then, for urban street, since hierarchy is not clear then classify by importance is the rule of thumb. For me the highest class of road in each city/town should not be higher than National Highway lead to that city/town.
In this question: https://help.openstreetmap.org/questions/20194/urban-highway-classification there are some good point. primary/secondary/tertiary road in each city/town is classified by importance of them for travel from one part of the city to another. And, dead end of each class should be avoided, but can occur, each class should connect to same class or higher class highway.