In Drury Displays, Inc. v. Brown (Ill.Ct.App. 1999) 306 Ill. App. 3d 1160, 715 N.E.2d 1230, 240 Ill. Dec. 173, the plaintiff was issued a permit for a billboard and completed its construction at a cost of over $ 49,000, then the department revoked the permit on the ground the department issued it in error, because the billboard was within 500 feet of another location as to which an earlier permit had been issued to another owner. The appellate court affirmed the trial court's summary judgment for a writ of mandamus compelling the department to reissue the permit, on the ground of estoppel. It stated that under Illinois law a party who detrimentally relied on conduct of a government agency and would suffer a substantial loss if the agency were permitted to retract its previous representation could invoke estoppel against the government, and all those elements were present. (Id. at pp. 1234-1235.)