Taking American Canyon as our point of interest, the crimes at Lake Herman Road and Blue Rock Springs would obviously employ a 'buffer zone' to his residence, and both occurring on a Friday (9-5 job), would also bring into play the 'least effort principle', where he is likely to travel less distance from his residence on a day in which he has worked. This would be the case in both of these crimes. But time and distance may have been paramount to the Zodiac Killer, plotting precisely how far he was prepared to travel, in conjunction with how long it would take.
American Canyon to Blue Rock Springs is approximately 8 miles or 15 minutes traveling time by vehicle.
The Zodiac Killer's buffer zones would be within 4 minutes of each other during the first two crimes.
His next crimes would be on Saturday, at Lake Berryessa and Presidio Heights. Assuming this was a non-working day, the least effort principle would apply less constraint to his movements. Not limited by time he can now travel further afield, which is exactly what he did. But would time and distance yet again come into play in his plans.
American Canyon to Presidio Heights is approximately 48 miles or 1 hr 12 minutes traveling time by vehicle.
American Canyon to Lake Berryessa is approximately 38 miles or 1 hr 09 minutes traveling time by vehicle.
The buffer zone difference in these two crimes is a mere 3 minutes. It was only 4 minutes in Vallejo and Benicia.
From American Canyon to the Springs and Tuolumne payphone is a 16 minute journey. From American Canyon to the 1231 Main Street payphone is a 26 minute journey.
The buffer zones from American Canyon to Lake Herman Road, Blue Rock Springs and the Springs and Tuolumne payphone are 19 minutes, 15 minutes and 16 minutes respectively.
The buffer zone in terms of minutes from the Knoxville Road to the Napa payphone would be 45 minutes, if the above scenario was correct. In other words, the Zodiac Killer allowed 45 minutes to elapse before calling the police. After the Blue Rock Springs attack on Michael Mageau and Darlene Ferrin he waited 40 minutes, only a 5 minute difference. It seemed like everything he did was based on the clock. The 4 minute difference to his Friday murder sites. The 3 minute difference to his Saturday murder sites. The 5 minute difference between the second and third crime scene to each phone call. A total of only 12 minutes.
If we include the time difference to each payphone from American Canyon, which is 10 minutes, that gives us a grand total of 22 minutes over four crime scenes, based on a Monday to Friday, 9-5 job, using the criteria of the 'least effort principle'.
But only if the Zodiac Killer lived in American Canyon.