Address German Umwelt zones in routing

Most cities in Germany have Umwelt zones where specific types of cars are not permitted to enter (i.e. vehicles with diesel engines). It would be useful to reflect this information somehow in the routing.

One item that could be added is an option to avoid such areas from routes when the user selects (i.e. and avoidable option). Another aspect could be to add warning messages to routes when they contain ways inside of these umwelt zones.

No files, tests, or entry points are named in the issue. Start by locating existing route-avoidance options and route warning handling, then clarify whether the expected result is an avoidable Umwelt-zone option, warnings on affected routes, or both, with acceptance criteria for each.

Written by the indexing model from the issue text.