Time is much more important than distance.
It’s probably best for labor and service if your drivers never take more than 20 minutes round-trip on any single delivery (worst case, single run to the far edge of the area, no other deliveries going that direction–8-9 minutes there, a few minutes to make the delivery, 8-9 minutes back).
You’ve got to pay $3 or more in labor/taxes, plus whatever you pay for vehicle reimbursement, when a driver is gone for 20-25 minutes. If it’s a only a $15 order, it’s not worth it for a driver to be gone longer.
I’ve noticed that a lot of Pizza Hut and Domino’s stores in my metro area are closing and consolidating delivery areas. They’re fools, if you ask me. Poor food that’s hardly inexpensive and ever-worsening service to boot? No wonder they can’t seem to reverse their slide into mediocrity.