[Openstreetmap] key values to represent restricted turns

[Simon Hewison]

Now that we have a selection of usable editors that understand keys and 
values, I've been thinking of ways of representing things that would 
affect a vehicle routing algorithm.

On a street segment, we need a "oneway=yes", and "class=motorway" can 
infer "oneway=yes", but none of the editors yet cab represent segment 
direction (e.g. with flow arrows), with the option of correcting which 
direction the traffic is supposed to flow.

Another useful node attribute would be something that denotes 
no-left-turn, or no-right turn. Whilst it's obvious that you cannot turn 
the wrong way down a one way street, definition of "no-left-turn" or 
"no-right-turn" is a little more tricky, as it can depend on which 
direction you approach the junction.

My suggestion is that "no-right-turn=yes" should mean any turn greater 
than 30 degrees to the right of straight ahead from the approach 
direction should be prohibited. Likewise in the opposite direction for 
"no-left-turn=yes". If this is on a corner, you may need to insert extra 
nodes.

If anyone can think of any better way of representing such constructs, 
please discuss it here.

-- 
Simon Hewison