In accordance with legal codes and custom in Hong Kong, once any signal higher than No. 3 is issued, all government agencies shut down their operations, schools immediately cease classes, financial markets and most of the private sector close their doors. Non-essential workers are released from work in a staggered manner so as to avoid overwhelming public transportation. Public transit agencies that operate on the sea or on surface streets and highways generally continue to operate initially but may cease operation at short notice. Employers who require staff to remain on duty during a No. 8 or higher signal must provide a safe work environment and shelter after work should transportation be unavailable. Such arrangements must be worked out beforehand between the employer and employee. In order to minimise disruption, the Hong Kong Observatory has, since 1987, observed a practice of issuing warnings two hours in advance of the issuance of the No. 8 warning signal, though such warnings of warnings are not attended by any system of numbering such as to indicate the relative imminence of issuance of the typhoon warning signal itself.