They have to issue a signal that covers the whole territory, not just built up areas. From teh HKO website:
Tropical Cyclone Warnings
Bulletin issued at 07:45 HKT 14/Aug/2013
Tropical Cyclone Bulletin
Here is the latest Tropical Cyclone Bulletin issued by the Hong Kong Observatory.
The No. 8 Southeast Gale or Storm Signal is in force.
This means that winds with mean speeds of 63 kilometres per hour or more are expected from the southeast quarter.
At 8 a.m., Severe Typhoon Utor was centred about 270 kilometres southwest of Hong Kong (near 20.5 degrees north 112.4 degrees east) and is forecast to move northwest at about 16 kilometres per hour towards the western coast of Guangdong.
Utor continued to move steadily towards the western coast of Guangdong in the morning. According to the present forecast track, Utor will be closest to Hong Kong during the day, skirting around 250 kilometres to the southwest of Hong Kong. Gales are still affecting the territory. The Gale or Storm Signal No. 8 will remain in force this morning.
"In the past hour, the maximum sustained winds recorded at Ngong Ping, Cheung Chau and Kai Tak were 110, 77 and 42 kilometres per hour with maximum gusts 139, 105 and 66 kilometres per hour respectively."