Reading over your comments about turn restrictions and the NY state code, I feel like we will end up with tortuous reasoning and miss the mark. Stepping way back and ignoring legal details, it seems like we need a schema to express what kinds of people/vehicles/etc. may do what, and a way to use that data in routers. An important case is transforming OSM data to GPS receivers, especially Garmin, and another case, which will perhaps become dominant, is a purpose-written program (e.g. for Android).
Given that goal, I lean towards encoding access rather than turn restrictions, because emergency U-turn places on interstates are not really well modeled with turn restrictions. If there were a no-U-turn restrictions, and someone pulled off (left), stopped, and pulled back on, I would expect the state police to frown upon that. In my view, any use of those areas by other than police/fire/etc. will be viewed as improper. I can certainly see the point that no-left-turn onto the way, no-left-turn off the way, and two no-u-turn highway-dirt-highway in both directions will work, but access=emergency on the way is far simpler. I lean to access on the restricted ways partly because I believe that will get translated to garmin format (mkgmap) better than turn restrictions. That may not be a valid long-term argument, but there is in general merit in simplicity. The access=yes/no/private/destination/emergency tag is a bit of a mess anyway, in theory, in my view. It started out talking about whether something was or wasn't a public right of way, and then overloaded a number of things. While one could argue that a fire engine responding to a call is obligated to obey no trespassing signs I'm quite sure that they will be ignored. So I would tag the ways access=emergency, and in translating to garmin convert access=(anything but yes) to access=emergency (assuming a working emergency mode, which is assuming a lot, but some models probably have it). I'm left with wondering about how to distinguish what's physically possible with what's prohibited. I know of one intersection (Mass Ave and Memorial Drive in cambrige), where heading east on Memorial Drive, there is an underpass, but if that isn't taken, you can turn right on Mass Ave, but you can't go straight or left because of jersey barriers. (I've seen people try, and dialed 911 for it many times....) Now, an ambulance could turn left, drive the wrong way on the one-way half of the street, and recover to the right after getting beyond, but that's a bit different. So I guess if the geometry reflects what is possible and the turn restrictions reflect what's prohibited but possible, this works. I hope this qualifies for not trolling :-)
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