A recent survey in Germany studied the locations of serial killer victims in respect to the killer's home residence and found that 63% of the time the killer lived within a 6 mile traveling distance to the crime scene. So putting all these factors together we could expect the Zodiac Killer's first murders at Lake Herman Road on December 20th 1968 to be only a matter of a few miles from his house. Bearing in mind the timeline that night, given by eyewitnesses Stella Medeiros and James Owen, it was determined the Zodiac Killer after brutally murdering David Faraday and Betty Lou Jensen must have driven east after the commission of the crime and headed toward Benicia. Using this as our guide, there may be a good chance that if you were to travel by vehicle from the Lake Herman Road turnout toward Benicia in your car, you may be near or have already passed the home of the Zodiac Killer, within or close to this six mile journey.
If we take the 'least effort principle', 'buffer zone' and the time constraints of his occupation into account, it makes San Francisco an unlikely home for the Zodiac Killer, assuming he remained in one location throughout his four attacks. If we take the mileage from the nearest point in San Francisco to the Lake Herman Road turnout, it is approximately 38 miles. See here. Therefore, the geographic profiling principle would be turned on its head. The Zodiac Killer would have finished work on Friday and have been prepared to throw the 'least effort principle' out of the window and create his initial 'buffer zone' at 38 miles or more when carrying out the Lake Herman Road attack. This would be followed by a 44 miles or more 'buffer zone' at Blue Rock Springs, and then he would have traveled even further, a distance of 80 miles to Lake Berryessa for his third attack, before finishing closest to his home residence with the murder of Paul Stine on October 11th 1969. It would mean he was committing three crimes in excess of 38 miles away, in less familiar areas, such as the rarely used Lake Herman Road, with less escape routes, as his opening gambit. In addition, he was prepared to expose himself even further by traveling on foot in a built up area, having just shot a taxicab driver and securing a 'trophy', all within the nearest point to his home residence.
The first two attacks also suggested the killer must have had prior knowledge that Lake Herman Road and Blue Rock Springs Park were popular haunts for courting couples, unlikely to have been known by a native of San Francisco - unless he worked in the locality of Vallejo or Benicia, or had previously lived there.
Furthermore, the unexplained 40 minutes after the Blue Rock Springs attack raises questions, bearing in mind the payphone was only 10 minutes journey time from the crime scene. If the killer had lived in San Francisco, he could have made this phone call anywhere en route with little risk. There would have been no reason for him to hang around Vallejo, spending 30 minutes longer than necessary to place the call to Nancy Slover, with all the risks this would have entailed.
Another geographic profiling term is 'distance decay', that states the farther you travel away from the killer's comfort zone (their area of familiarity), the less likely they are to kill. Although we only have four crime scenes on our map above, it nevertheless would exhibit this trait, based on a premise of our killer living close to both Lake Herman Road and Blue Rock Springs Park. The cluster of pins in the map would therefore decay the further we traveled from the murderer's base location. It is hard to call the first two crimes a cluster, but so far they are more suggestive of a warm zone than San Francisco regarding the circumference of our circle. It is true that many Zodiac letters were mailed from San Francisco, but this is more than likely where he worked, as opposed to where he lived, and likely mailed these letters on his journey to and from work. Had he finished work at 5:00 pm on Friday afternoon in San Francisco and arrived back in Benicia or Vallejo sometime approaching 6:00 pm, the 'least effort principle' would now kick in. This was ultimately reflected in his choice of locations on December 20th 1968 and July 4th 1969. In fact, the further away his workplace was, the greater this 'least effort principle' would apply, not only because of tiredness, but the time constraints of traveling further afield. In all probability the Zodiac Killer lived in either Vallejo or Benicia - which leaves one final question.
Bearing in mind the double murder at Lake Herman Road on December 20th 1968 at the dead of night, with the killer having fired off at least ten rounds, probably having picked up biological transference from the brutal execution of David Faraday, carrying the smoking gun, having limited escape routes, and in all likelihood, heading for the safety of home, did he turn left to Vallejo, or right to Benicia. This may be the answer we are looking for.