Calamiteitenroutes and U-routes
Calamiteitenroutes in Belgium and U-routes (Uitwijkroutes) in the Netherlands are permanent routes that are used to reroute traffic in case of a closure of the main road.
The route identification can be added as alternative name to the street name.
The road type should be at least Minor Highway.
Calamiteitenroute
The Belgium Calamiteitenroutes are marked with a single character, like F.
More information on Calamiteitenroutes see the Belgium site wegenenverkeer.be.
U-route
The Dutch routes are marked with an "U" plus number, like U27.
More information on U-routes can be found in the Dutch Wegenwiki.