Different strategies for locating systems of sirens exist in different locales across the nation.  In Ann Arbor, and elsewhere, sirens noise is designed to alert citizens in the outdoors.  Citizens who are indoors may hear the sirens but the requirement is that people outdoors be able to hear them.  Thus, spacing requirements between sirens becomes important.  When there are barriers to overcome (all else being equal), such as topography, buildings and street noise, one might expect sirens to be required to be more closely spaced than in flat, open countryside.  Indeed, a brief review of municipal requirements on the world wide web reveals that Oakland County, Michigan views each siren to be capable of covering about a one mile radius.  The Baltimore City Fire Department selects spacing at 3200 feet.

The sequence of animaps below, of Ann Arbor, suggests a locational strategy for pinpointing positions for new sirens.

• the red dots show the location of the existing system of sirens.
• The light green circles are buffers of radius 3200 feet, the Baltimore standard.  Employing the Baltimore standard provides continuous central coverage with gaps at the perimeter.
• The light yellow circles are buffers of radius one mile, the Oakland County standard.  Employing the Oakland County standard provides a continuous block of coverage.  As new areas come in to the city in 2007, as per boundary agreements, new sirens will need to be added to maintain coverage.
• The red outlines of polygons, in a sort of bubble foam, are outlines of the Dirichlet tesselation on the fire stations.  The Dirichlet polygons are mutually exclusive and cover the entire area in the one mile buffer.  Pick any point within the one mile buffer.  Note which Dirichlet polygon contains it.  Thus, the siren in the same Dirichlet polygon as the selected point is the siren closest to that selected point.  Each Dirichlet polygon contains all the points closest to the siren in that polygon.

Figure 1.  Red dots show existing tornado siren locations.  Green circles use the coverage radius employed by Baltimore, MD; yellow circles use the coverage radius employed by Oakland County, MI.

• The red dots and the Dirichlet tesselation are as above.
• Successive buffers have radii of 1000, 2000, 3000, 4000, and 5000 feet.

Figure 2.  Spacing between successive buffers of sirens is 1000 feet.

• The red dots and the Dirichlet tesselation are as above. The white background has been removed, inverting the emphasis on the road network.
• Successive buffers have radii of 1000, 2000, 3000, 4000, and 5000 feet.
• Streets enter the picture along with buffers, showing zones of connectivity and perhaps suggesting emergency routes in the 3000 or 4000 foot buffer level.  There is a northwest arterial that is entirely contained within the 4000 foot buffer.  On the east side, routes through the southeast/central (Ann Arbor Hills) area show strong coverage.

Figure 3.  In this view, connectivity of the road network, already within earshot of existing tornado sirens, is emphasized.

Finally, where might one consider locating new sirens (Figure 4)?

• The 5000 foot view shows a gap in coverage just south of Pioneer High School, east to the U. of M. golf course.
• Within the Dirichlet tesselation, highest priority might therefore (all other things being equal) be given to putting a siren in the gap; indeed, golfers are an important target population!
• Outside the Dirichlet tesselation, highest priority might therefore be given to the gap at the right edge of the tesselation that is within the freeway ring but is as yet uncovered by a siren.  The location for a new siren was found by digitizing the uncovered area, calculating the centroid of the digitized region, and then using the centroid as the proposed siren location.  In implementation, it is likely that actual position will not follow centroid location exactly as one factors in property rights, ease of siren maintenance/access, and so forth.

Figure 4.  Cyan concentric circles targe locations for two new tornado sirens.

• Matthew Naud, Environmental Coordination Services, Director, City of Ann Arbor;
• Merle Johnson, Information Technology Services, City of Ann Arbor;
• Adele ElAyoubi, Neighborhood Watch Coordinator, City of Ann Arbor Police Department;
• Karen Hart, Planning Director, City of Ann Arbor.

Oakland County, Michigan
http://www.co.oakland.mi.us/ems/program_service/torn_siren.html
 

Baltimore, Maryland, Fire Department
http://www.ci.baltimore.md.us/government/fire/pr021016.html