Route over alternating oneways but not reversible ones, closes #2837.

- alternating: high frequency, route over them with penalty
- reversible: low frequency, do not route over them - time dependence

- http://wiki.openstreetmap.org/wiki/Tag:oneway%3Dreversible
- http://wiki.openstreetmap.org/wiki/Tag:oneway%3Dalternating

This distinction was made at the Elbe-Labe Meetup in Dresden, with
accompanying Wiki pages and tagging scheme. Thanks all involed!

- https://github.com/Project-OSRM/osrm-backend/issues/2837
- http://wiki.openstreetmap.org/wiki/Key:oneway

profiles/car.lua

@@ -308,7 +308,9 @@ function way_function (way, result)
     return
   end
 
-  -- check if oneway tag is unsupported
+  -- Reversible oneways change direction with low frequency (think twice a day):
+  -- do not route over these at all at the moment because of time dependence.
+  -- Note: alternating (high frequency) oneways are handled below with penalty.
   local oneway = way:get_value_by_key("oneway")
   if oneway and "reversible" == oneway then
     return
@@ -584,6 +586,18 @@ function way_function (way, result)
     result.backward_speed = math.min(penalized_speed, scaled_speed)
   end
 
+  -- Handle high frequency reversible oneways (think traffic signal controlled, changing direction every 15 minutes).
+  -- Scaling speed to take average waiting time into account plus some more for start / stop.
+  if oneway and "alternating" == oneway then
+    local scaling_factor = 0.4
+    if result.forward_speed ~= math.huge then
+      result.forward_speed = result.forward_speed * scaling_factor
+    end
+    if result.backward_speed ~= math.huge then
+      result.backward_speed = result.backward_speed * scaling_factor
+    end
+  end
+
   -- only allow this road as start point if it not a ferry
   result.is_startpoint = result.forward_mode == mode.driving or result.backward_mode == mode.driving
 end