The fault seems to be memory related. When driving a route with a large number of decals in it, as you go along decals are loaded. It appears that decals are not removed from memory and so after a while you may run out of resources and cause a crash.
As decals are meant to only be left in during the design stage and as it takes approx 40 miles of driving (in my experiance) before the error happens, it is still possible to test routes with decals in place for large distances.
Decals should be removed from a route after objects have been placed.