In terms of being short of drivers, how far do you go? How much spare coverage do you have?
If you have a small depot at the far end of the route, which has 8 duties per day (4 early and 4 late), how many staff do you roster each day to ensure that no matter what happens all duties are always covered in all circumstances all the time, without the need for any occasional overtime?
Lets say you have 100% spare cover. So each day you have 4 early duties and 4 early spare men, just in case. The same on lates. So that's 16 staff per day on duty to cover 8 jobs. Not only is that extremely expensive, it is still no guarantee that everything can be covered. Lets say that of the 4 late staff who have booked duties, 1 is sick (he phoned up this morning to say he was ill) and two are on restricted duties due to prescribed medication, getting back to work with a minder after a fatality, or a host of other reasons. Never mind, you have 4 spare men to cover those 3 uncovered jobs. But wait, of those 4 spare men 2 are on annual leave and 1 is actually a vacancy because someone left to work on freight 2 weeks ago having worked his 6 months notice, but as it takes 18 months to recruit and train a replacement, his replacement hasn't started yet. So you only have 1 spare man to cover 3 uncovered duties. So you still have 2 uncovered duties. How do you cover them? Yet in the morning you had 4 rostered staff and 4 staff sat spare, but they can't be moved to late shifts as you can only move someone 3 hours either way, and they are on an early start the next day anyway.
So do you have even more spare staff, who would, for most of the time, be sat around twiddling their thumbs getting paid £47k per year for doing so. That sounds extremely wasteful to me.