The Fairfield letter has a threatened kill count of 9 in Sacramento, 8 in Oakland, 8 in Napa, 6 in Vallejo, 3 in Fairfield, 3 in San Francisco and 1 in San Jose. This would suggest that San Jose would be his furthest commute to kill, with his residence at a reasonable traveling distance to Sacramento, Oakland, Napa and to a slightly lesser extent Vallejo. The letter itself was postmarked Fairfield, which is telling, because Sacramento to the northeast of Fairfield is 37 miles, while Oakland to the south is 34 miles. Fairfield to Napa is 14 miles, and to Vallejo is 15 miles. Therefore, Fairfield is fairly equidistant to the areas targeted in the letter the most (Sacramento, Oakland, Napa and Vallejo). The two furthest from Fairfield are targeted the least in terms of a proposed kill count (central San Francisco and San Jose, 39 and 63 miles respectively). Does this tell us anything about the Zodiac Killer's residence based upon the distance decay model of geographic profiling. Distance decay is a geographical term which describes the effect of distance on cultural or spatial interactions. The distance decay effect states that the interaction between two locales declines as the distance between them increases. Once the distance is outside of the two locales' activity space, their interactions begin to decrease. The Zodiac Killer would therefore attack less frequently the further he moved away from his residence and comfort zone. However, this is not what interested me regarding the Fairfield letter.
"I-80 begins at an interchange with U.S. Route 101 (US 101) in San Francisco, and then crosses the San Francisco–Oakland Bay Bridge into Oakland. It then heads northeast through Vallejo, Sacramento, and the Sierra Nevada mountains before crossing into Nevada" Wikipedia.. Was this commuter route integral to the movements of the Zodiac Killer, such as a profession or pastime? His early correspondence was mailed to San Francisco and Vallejo, with Fairfield accounting for two postmarks on December 7th 1969 and December 16th 1969. If we include the December 10th 1969 and December 11th 1969 paste-up letters, dubbed the "Cancer" and "Leo" communications, then these were mailed from Sacramento to Sacramento, and Fairfield to San Francisco respectively. These communications were spanning Sacramento to San Francisco, taking in Fairfield, Vallejo and Oakland along the way - the primary hit zones identified in the December 16th Fairfield letter. The only exception being Napa, positioned only 7 miles off route to Interstate 80 at its nearest point. We will call this geographic profiling of the future.