Density is the quiet number behind almost every other routing problem. Two technicians can complete the same twelve stops on the same day and have completely different days: one drives 38 miles, the other 94. The second one is not slower. The route was built differently.
Why it matters
Every mile between stops is paid for twice — once in fuel and wear, once in the hour it takes out of the day. A route that loses four minutes between each stop loses roughly 45 minutes across a twelve-stop day. That is one more job you cannot book, every day, for every technician.
Density is also what makes a day predictable. Stops that sit close together absorb a late start or a long job. Stops that sit far apart pass the delay along, and the last customer of the day gets the apology.
How you notice it slipping
Miles per stop climbs while the job count stays flat. Technicians start asking which order to drive in rather than following the plan. The same street appears on two different days of the week. Overtime shows up on the days with the fewest jobs, which is the clearest sign of all.
What to do
Measure miles per completed stop by route and by week, not by month — a monthly average hides the day that went wrong. Then fix the cause rather than the symptom: density is usually destroyed upstream, by a booking promise or a territory boundary, long before the optimiser sees it.