In previous articles we have explored the 'science' of geographic profiling to help narrow down a possible search field based on the locality of the four Zodiac crime scenes, using buffer zones and the least effort principle to facilitate this. Most (but not all serial killers) do not hunt on their own doorstep, because they leave a distinct distance from their home to the scene of the crime which is individual to each person, but often serial killers begin their murders close to home before venturing outwards. This pattern of human behavior has been used in many cases down the years and was employed with great accuracy in the case of the Green River Killer, Gary Ridgway. Of course, the following is not without assumptions, but if we take it that our killer, for example, had a Monday to Friday, 9 to 5 profession, then using his time constraints and the least effort principle, it would not be unusual for any murders committed on these days to be relatively close to home. Conversely, any murders on a Saturday are not subject to these same constraints (as it is a rest day), thereby allowing greater flexibility to the murderer. So it's not surprising that on a Saturday the Zodiac Killer would likely increase his buffer zone to commit his third and fourth attacks, which is exactly what he did. However, this would be making the assumption the killer lived somewhere in the Vallejo or Benicia regions, relatively, but not too close to the first two crime scenes. There is good reason to believe this based on everything he did, including his letter writing and likely exit routes after the Lake Herman Road, Blue Rock Springs Park and Lake Berryessa attacks.
The following will triangulate on a map the exact locations of his four attacks, encompassing his two phone calls and confirmed Zodiac letters, all of which fall within an approximate one-half of a circle, drawn around his furthest two crimes, in what's known as the Canter and Larkin circle theory of environmental range. In other words, the greatest distance our killer seemed prepared to go.
What is noticeable, is that all these locations drop within a 180-degree segment of the circle, the center of which literally sits over the first two crime scenes. If we remove the Pleasanton mailing, the focus narrows to literally a third of a circle, giving us a very narrow field of view of the Zodiac's entire known activity.
The 'hot zone' on this map sits over the Vallejo and American Canyon region, indicating the Zodiac Killer likely lived in this region and had a environmental range inside of 50 miles. If the Zodiac Killer lived in Vallejo and worked in San Francisco, then mailing his letters away from his home residence makes logical sense. The San Rafael, Alamada County and Oakland areas are on a near direct traveling route, either side of the Bay Area, to and from San Francisco - if indeed the Zodiac Killer lived in Vallejo.
The killer made two phone calls, possibly having familiarity with the location of the Springs and Tuolumne intersection payphone, as it was an area he likely passed after his first two attacks, heading towards Columbus Parkway on both occasions and along Springs Road to the heart of Vallejo. In the second instance, a line drawn from Lake Berryessa to 1231 Main Street continues directly on to our center of focus, downtown Vallejo or the American Canyon region. These areas appeared to be the Zodiac Killer's comfort zone, but the primary focus always tends to a narrow region within 7 miles traveling distance from the first murders at Lake Herman Road on December 20th 1968 - a remote, little used, two-lane stretch of road he appeared to know only too well.