As has often been noted, the highway tag “is a very general and sometimes vague description of the importance of the highway for the road grid.” Importance, however, is difficult to objectively quantify. Official/legal classifications usually serve as an adequate guide, but not all types of highways have classifications.
For the benefit of those unfamiliar with all highway types here’s a recap:
Most urban streets would legally be local roads. These local roads (excluding those in Bangkok and Pattaya) are being registered, and given ref numbers and road grades, but the grades are based on physical condition and the refs don’t reflect their importance. (See the registration page for Chiang Mai City Municipality, for example.) I’m not sure whether tagging all local roads as highway=unclassified would be appropriate for other urban areas, but it would be totally inadequate in Bangkok. I’ve asked on the help page for advice, and there have been some good replies.
For Bangkok, I think the BMA’s Traffic and Transportation Department’s statistics reports may serve as a good starting point. They have tables showing the amount of traffic and travel times on “important roads” in the city. However, different editions show different lists of roads, so it doesn’t seem to be an official designation.
The roads usually included in said tables are: Kaset–Nawamin, Ngam Wong Wan, Charansanitwong, Charoen Krung, Charoen Nakhon, Chaeng Watthana, Narathiwat Ratchanakharin, Nawamin, Borommaratchachonnani, Bangna–Trat, Prachathipok, Prachasongkhro, Phaya Thai, Rama IX, Rama I, Rama II, Rama III, Rama IV, Rama V, Rama VI, Phahonyothin, Phatthanakan, Phitsanulok, Phetchaburi, Yaowarat, Ratchadaphisek, Ratchadamnoen, Ratchadamri, Ratchawithi, Ratchasima, Ramkhamhaeng, Ram Inthra, Lat Phrao, Vibhavadi Rangsit, Srinagarindra, Si Burapha, Si Ayutthaya, Sawankhalok, Sathon, Samsen, Si Lom, Sukhumvit, Surasak, Seri Thai, Lan Luang, Arun Amarin, Asok Din Daeng, and Inphithak–Phetkasem.
These roads should probably be at least highway=secondary, as they link different parts of the city. How many of them should be primary, I’m not quite sure. Also, as I said on the wiki talk page, my usual rule of thumb is to use at least highway=tertiary for thanon and residential or unclassified for sois. I would like to know what others think of this.